In 3 Nephi 11:35-36, Jesus identified three who would witness the making of the covenant on this occasion, the Father, Christ himself, and the Holy Ghost. According to John Welch, from the Old Testament we know that it is in the mouth of two or three witnesses that all things are to be established. One may think also of the three messengers who visited Abraham, or the calling of witnesses in Joshua 24:15, where Joshua says, "Choose ye this day whom ye will serve" and then points out that the people themselves on that occasion were witnesses of what they had done. That is an important element of most covenant making--that it be done in the presence of witnesses--and certainly on this occasion we have that condition fulfilled as well. [John W. Welch, "Christ at the Nephite Temple," in Teachings of the Book of Mormon, Semester 4, p. 135]
Filling a covenantal role that is familiar from several other occurrences in scripture (d.g., Genesis 18:2; Deuteronomy 4:26; 19:15; 2 Nephi 11:3), these three stand together at the commencement of this dispensation of the new law to the Nephties to witness of the gospel. [John W. Welch, The Sermon at the Temple and the Sermon on the Mount, p. 41]