According to Lee Donaldson, the ancient covenantal pattern of having something done three times or three ways is fulfilled in multiple ways in the Book of Mormon. Moroni, in the middle of his abridgment of the Jaredite record, prophesies that,
“in the mouth of three witnesses shall these things be established; and the testimony of three, and this work, in the which shall be shown forth the power of God and also his word of which the Father, and the Son, and the Holy Ghost bear record--and all this shall stand as a testimony against the world at the last day.” (Ether 5:4)
The “new covenant, even the Book of Mormon ”(D&C 84:57) has been established like other covenants by having three witnesses. Traditionally, we think of the three witnesses to the Book of Mormon as Oliver Cowdery, Martin Harris, and David Whitmer. Certainly, they do stand as latter-day witnesses to the divine origin of the Book of Mormon. However, there are additional sets of three witnesses to this sacred record. Moroni declared that the Father, and the Son, and the Holy Ghost bear record“ (Ether 5:4). Also, three branches of the House of Israel, the Jews, the Nephites, and the lost tribes, each kept a record, and each branch will eventually receive the other’s records (2 Nephi 29:12-14). However, another set of three witnesses, the three sets of plates that make up the Book of Mormon, is in the forefront in establishing the Book of Mormon. The plates of Nephi, the plates of Mormon, and those of Ether stand as vital components in establishing the Book of Mormon covenant. Each of these plates contains each of the six elements of the ancient covenant and also serves to illustrate the ramifications of the covenant to a different nation. [Lee L. Donaldson, ”The Plates of Ether and the Covenant of the Book of Mormon," in The Book of Mormon: Fourth Nephi through Moroni, From Zion to Destruction, p. 74]