The gold plates will be shown to those called to assist in bringing forth the record, including eleven witnesses, and to three witnesses (compare 2 Nephi 11:3) specifically by the miraculous power of God, and their testimony will be a sure, irrefutable one. In addition to those witnesses, two other witnesses will stand in the latter days to affirm the truth of the Book of Mormon record: the book itself and Moroni himself, whom Joseph Smith would see face-to-face.
Witnesses often come in multiples of three: the Three Witnesses of the ancient American records; the Bible, the Book of Mormon, and the Doctrine and Covenants; and the Father, the Son, and the Holy Ghost.