By June 1829 Martin Harris, Oliver Cowdery, and David Whitmer had verbalized a desire to be the special three witnesses alluded to in the Book of Mormon.
D&C 17 records a revelation affirming their roles as witnesses and was given to Joseph Smith through a seer stone he apparently found while digging a well in 1822.
As witnesses, the three were very different. Martin Harris was zealous, impetuous, and even a bit eccentric. Oliver Cowdery was an intellectual. And David Whitmer was regarded as clear-thinking, down-to-earth, and honest.
David Whitmer was, perhaps, the strongest witness because he lived so long, never wavered in his testimony of the vision, and gave several newspaper interviews that give us additional details regarding the experience. David reported seeing several plates, the sword of Laban, the Liahona, and the Urim and Thummim.
Joseph Smith was understandably relieved to have others to testify of the existence of the plates. Larry Morris concludes that the experience of the Three Witnesses was both an empirical and spiritual experience.
Join Nick Galieti of LDS Perspectives Podcast as he interviews Larry Morris as part of the Revelations in Context podcast series.