A revelation given to Joseph Smith (D&C 17) confirmed that Oliver Cowdery, David Whitmer, and Martin Harris would be the Three Witnesses. They met together on 28 June 1829 in the woods near the Whitmer Farm outside Fayette, New York and began praying for this promised divine manifestation. 
The "Testimony of Three Witnesses" summarizes the supernatural event that followed, when an angel appeared and showed them the plates and engravings and they heard the Lord declare that the Book of Mormon was "translated by the gift and power of God." They said that the same divine voice "commanded us that we should bear record of it."
Though each of the Three Witnesses was eventually excommunicated from the Church (two later returned), none ever denied or retracted his published testimony. Each reaffirmed at every opportunity the veracity of his testimony and the reality of what he had seen and experienced. Of the perhaps 200 recorded interviews with the Three Witnesses, a significant percentage stress the spiritual intensity of the witnesses as they described the angel and the plates. By themselves, the Prophet's reputation and claims were vulnerable, but the testimony of additional reputable, solemn witnesses who shared a divine experience added credibility.
Be it known unto all nations, kindreds, tongues, and people, unto whom this work shall come: That we, through the grace of God the Father, and our Lord Jesus Christ, have seen the plates which contain this record, which is a record of the people of Nephi, and also of the Lamanites, their brethren, and also of the people of Jared, who came from the tower of which hath been spoken. And we also know that they have been translated by the gift and power of God, for his voice hath declared it unto us; wherefore we know of a surety that the work is true. And we also testify that we have seen the engravings which are upon the plates; and they have been shown unto us by the power of God, and not of man. And we declare with words of soberness, that an angel of God came down from heaven, and he brought and laid before our eyes, that we beheld and saw the plates, and the engravings thereon; and we know that it is by the grace of God the Father, and our Lord Jesus Christ, that we beheld and bear record that these things are true. And it is marvelous in our eyes. Nevertheless, the voice of the Lord commanded us that we should bear record of it; wherefore, to be obedient unto the commandments of God, we bear testimony of these things. And we know that if we are faithful in Christ, we shall rid our garments of the blood of all men, and be found spotless before the judgment-seat of Christ, and shall dwell with him eternally in the heavens. And the honor be to the Father, and to the Son, and to the Holy Ghost, which is one God. Amen.
Law of Witnesses
The Book of Mormon is an additional set of sacred prophetic writings adopted by the The Church of Jesus Christ of Latter-day Saints (as well as it many offshoots, collectively referred to as Mormonism) as both Holy Scripture and Another Witness of the Divinity of Jesus Christ.
An important element to proving the divine authenticity of this scripture is The Testimony of Latter-day Witnesses. Their experiences with this book have been recorded for all mankind.
This is the third time I am coming to you. In the mouth of two or three witnesses shall every word be established. (II Corinthians 13:1)
This work is attributed to ancient Jewish prophets that fled the destruction of Jerusalem in about 600 B.C. and came with their families to settle in the Americas. They recorded their prophecies, teachings, wisdoms and history upon golden plates and hid them in the ground.
In 1823, the Mormon prophet and church founder Joseph Smith declared that an angel named Moroni revealed these plates and their hiding place to him. The angel declared that those who wrote this record were additional ancient prophets of Jesus Christ. The record itself is frequently referred to as "Another Witness of Jesus Christ" and is to support (not detract) from the testaments in the Holy Bible. This angel also gave him tools by which he could translate them. In 1827 he was able to take these golden plates to his home and begin the translation process, taking over 3 years, and eventually published to the world in 1830 as the Book of Mormon. At the same time he and his friends founded the church based on its teachings.
When Joseph Smith first obtained the gold plates, he was told to show them to no one. During the course of translation, he learned that of the need for special witnesses to affirm the truthfulness of this record. Consisting of men known for truthfulness and sobriety, who would come to know, by the power of God, "that these things are true" and that several besides himself would see the plates and testify to their existence (Ether 5:2-4; 2 Ne. 27:12-13; D&C 5:11-13).
One oddity about the Three Witnesses, is that all three at one point or another left the church. But the reason for their departure was dispute with Joseph Smith or methods of church governance and/or the question on the doctrine of polygamy. At no time to they recant their testimony of the Book of Mormon, remaining faithful to the end.
Biographical links for the life story and additional witness statements for each of these three individuals:
- Final Testimonies of the Three Witnesses - New Era Magazine - Jan 2020.
Oliver Cowdery - a young school teacher who served as a scribe to Joseph Smith for much of the translation process. an important participant in the formative period of the Latter Day Saint movement between 1829 and 1836. He was the first baptized Latter Day Saint, one of the Three Witnesses of the Book of Mormon's golden plates, one of the first Latter Day Saint apostles, and the Second Elder of the church. In 1838, as Assistant President of the Church, Cowdery resigned and was excommunicated on charges of denying the faith. Then later in 1848, he returned to the Latter Day Saint movement.
- Oliver Cowdery - Biography, First Historian of The Church of Jesus Christ of Latter-day Saints
- The Testimony of Oliver Cowdery - Ensign Magazine - Dec 1996.
- 1844 Letters from Oliver Cowdery to W.W. Phelps - "On the Origin of the Book of Mormon and the Rise of the Church of Jesus Christ of Latter-day Saints".
- Oliver Cowdery - Familypedia Genealogy.
Near this time of the setting of the sun, Sabbath evening, April 5th, 1929, my natural eyes for the first time beheld this brother [Joseph Smith, Jun.]; he then resided in Harmony, Susquehanna county, Penn. On Monday, the 6th, I assisted him in arranging some business of a temporal nature, and on Tuesday, the 7th, commenced to write the Book of Mormon. These were days never to be forgotten; to sit under the sound of a voice dictated by the inspiration of heaven, awakened the utmost gratitude of this bosom! Day after day I continued, uninterrupted, to write from his mouth, as he translated with the Urim and Thummim, or as the Nephites would have sait, "interpreters," the history or record called "The Book of Mormon." (1844 letter from Oliver Cowdery to W.W. Phelps)
For, the question might be asked, have men authority to administer in the name of Christ who deny revelations, when his testimony is no less than the spirit of prophecy, and his religion based, built, and sustained by immediate revelations in all ages of the world when he has had a people on earth? (Ditto)
Martin Harris - Martin sold 151 acres of his property to pay the debt. In addition to this great financial sacrifice, Martin sacrificed much for the gospel of Jesus Christ. While at one time he was “one of the most socially and politically prominent members of the community,”5 his support of Joseph Smith and the Church “cost him his political office, his social position and ultimately helped lead to the dissolution of his marriage.”
- Martin Harris, The Great Benefactor - (Church History Museum)
- The Life of Martin Harris: Patterns of Humility and Repentance” - Ensign Magazine July 2012,
- The 116 Lost Manuscript Pages - The Friend, March 1997 (Children's Magazine of the Church)
- Publicatiton of the Book of Mormon - Liahona Magazine - Jan 2009. Illustrated retelling of the story of Martin Harris mortgaging his farm to pay for the printing of the Book of Mormon.
- The Witness: Martin Harris - One of Martin Harris’s greatest contributions to the Church, for which he should be honored for all time, was his financing the publication of the Book of Mormon. by Elder Dallin H. Oaks, Quorum of the Twelve Apostles, May 1999, Ensign Magazine.
- Clarkston Pageant - For many years, his direct descendants living in Clarkston UT, held a pageant to retell his story and to commemorate his testimony and other contributions towards the publishing of the Book of Mormon.
- Martin Harris - Familypedia Genealogy.
David Whitmer - an early adherent of the Latter Day Saint movement who eventually became the most interviewed of the Three Witnesses to the Book of Mormon's golden plates.
- True to the Book of Mormon: The Story of the Whitmer Family - Ensign Magazine - Feb 1989.
- David Whitmer Interviews - Restoration Witness - Amazon.com
- Authenticity of the Book of Mormon by David Whitmer - Early devotee to the Mormon church David Whitmer issues here his famous missive addressed to all the Christians on Earth. Testifying to the existence of the golden plates discovered by Joseph Smith in New York state, David Whitmer expounds upon the principles of Mormonism and encourages readers to heed his declaration. Whitmer offers here a sincere and frank reflection upon the LDS movement. Through life, he would become the most sought-after figure for comment on Mormon matters. As Mormons grew in number and influence, Whitmer's fame was maintained by his status as one of the 'Three Witnesses' to the emergence of the Book of Mormon's golden plates.
- Last Testimony of David Whitmer - FairMormon.org
- David Whitmer (1805-1888) - Familypedia Genealogy.
In 1881, David Whitmer published “A Proclamation” to refute claims that he had denied his testimony:
It having been represented...that I, in a conversation with [John Murphy] last summer, denied my testimony as one of the three witnesses to the “Book of Mormon.” To the end, therefore, that he may understand me now, if he did not then; and that the world may know the truth, I wish now, standing as it were, in the very sunset of life, and in the fear of God, once for all to make this public statement: That I have never at any time denied that testimony or any part thereof, which has so long since been published with that Book, as one of the three witnesses. Those who know me best, well know that I have always adhered to that testimony. And that no man may be misled or doubt my present views in regard to the same, I do again affirm the truth of all my statements, as then made and published.
Lucy Mack Smith Journal
The angel Moroni showed the gold plates to Joseph Smith, Oliver Cowdery, David Whitmer, and Martin Harris in the woods near the Whitmer home in Fayette, New York. Joseph’s parents were visiting the Whitmers at the time. Lucy Mack Smith (1775-1856), Joseph’s mother, described the effect this miraculous experience had on the witnesses:
“It was between three and four o’clock. Mrs. Whitmer and Mr. Smith and myself were sitting in a bedroom. I sat on the bedside. When Joseph came in, he threw himself down beside me. ‘Father! Mother!’ said he. ‘You do not know how happy I am. The Lord has caused the plates to be shown to three more besides me, who have also seen an angel and will have to testify to the truth of what I have said. For they know for themselves that I do not go about to deceive the people. And I do feel as though I was relieved of a dreadful burden, which was almost too much for me to endure. But they will now have to bear a part, and it does rejoice my soul that I am not any longer to be entirely alone in the world.’ Martin Harris then came in. He seemed almost overcome with excess of joy. He then testified to what he had seen and heard, as did also the others, Oliver and David. Their testimony was the same in substance as that contained in the Book of Mormon. …
“Martin Harris particularly seemed altogether unable to give vent to his feelings in words. He said, ‘I have now seen an angel from Heaven who has of a surety testified of the truth of all that I have heard concerning the record, and my eyes have beheld him. I have also looked upon the plates and handled them with my hands and can testify of the same to the whole world. But I have received for myself a witness that words cannot express, that no tongue can describe, and I bless God in the sincerity of my soul that he has condescended to make me, even me, a witness of the greatness of his work and designs in behalf [of] the children of men.’ Oliver and David also joined with him in solemn praises to God for his goodness and mercy. We returned home [to Palmyra, New York,] the next day a cheerful, rejoicing little company.”
Additional testimony given by the Three Witnesses:
- Elder David H. Cannon, when returning from a two-year mission to England in 1861, stopped on the way home and visited with Martin Harris, David Whitmer and visited the gravesite of Oliver Cowdery. Both Martin Harris and David Whitmer gave testimony to him of the angel showing them the plates from which the Book of Mormon was translated. And he heard reports that Oliver Cowdery bore witness to the same and had rejoined the Church before his death.
- Historicity of the Book of Mormon
- Witnesses to the Gold Plates - Stories of additional people who have seen the original Gold Plates.
- Book of Mormon
- Book of Mormon - LDS.org, official website - available in many languages.
- The Gift and Power of God - Ch 6 of the LDS Church History: Saints.
- Mormon Pioneers
- Book of Mormon, Witnesses of - Religion.Wikia.org
- Oliver Cowdery (1806-1850)
- David Whitmer (1805-1888)
- Martin Harris (1783-1875)
- The Book of Mormon on Wikisource
- Witnesses of the Book of Mormon - LDS New Era April 2013 -
- Book of Mormon Witnesses - BYU Encyclopedia of Mormonism (Anderson, Richard Lloyd 1992)
- Why were 3 Witnesses Chosen - Video prepared by Book of Mormon Central
- True to the Book of Mormon: The Whitmer Family - LDS Ensign Magazine - Feb 1989.
- A Witness and A Warning: A Modern-Day Prophet Testifies of the Book of Mormon Paperback – April 6, 2009 - book by Ezra Taft Benson, 13th President of the LDS Church
- A Witness and a Warning - Testimony by Ezra Taft Benson, 13
- The Witness: Martin Harris - LDS Ensign May 1999 - Dallin H Oaks (Video/Text)
- The Whitmers: A Family that Nourished the Church - LDS Ensign Aug 1979.
- FindAGrave Memorial #12061 - David Whitmer - Richmond Cemetery.
- Stories of Martin Harris
- FindAGrave Memorial #12061.
- [Lucy Mack Smith, History, 1844–1845, book 8, page 11–book 9, page 1, josephsmithpapers.org; capitalization and punctuation modernized.]