According to Stan Larson, while it is true that both 2 Nephi 27:12 and Ether 5:2-4 speak of the three witnesses, Joseph Smith specifies that it was the former passage that brought the idea of witnesses to the attention of those who were assisting him. (See the "Manuscript History of Joseph Smith," p. 23, in which the reference is to page 110 of the 1830 edition, which was the eleventh chapter of 2 Nephi in that edition and corresponds to 2 Nephi 27 of the present edition.) [Stan Larson, "A Most Sacred Possession," The Ensign, September 1977, p. 91]
2 Nephi 27:12 Three witnesses shall behold it ([Illustration]): Oliver Cowdery, David Whitmer, Martin Harris. The Lord provided witnesses to testify and establish His word. The Three Witnesses--Oliver Cowdery, David Whitmer, and Martin harris--testified that an angel of God showed them the gold plates of the Book of Mormon. [Thomas R. Valletta ed., The Book of Mormon for Latter-day Saint Families, 1999, p. 137]