Why three witnesses? Why not one? Why not two?
The reference to witnesses comes from Deuteronomy:
Deuteronomy 17:6
6 At the mouth of two witnesses, or three witnesses, shall he that is worthy of death be put to death; but at the mouth of one witness he shall not be put to death.
The witnesses in Deuteronomy are for a very serious accusation. The witnesses stand between an accused and death, and such a serious accusation required more than a single witness to establish the guilt and impose such a final penalty. In the case of the coming forth of God’s work, the law of witnesses is again appropriate. However, while two is the minimum, three is better, as it implies an even wider establishment of truth. Thus Moroni mentions three so that the law of witnesses will have full measure. The three come from the scriptural passage, and are the “full set” of witnesses, the most sure witness of an important action.
Of course there are later to be eight more called. Does this violate Moroni’s indication that three would be called? It is certainly more than the three, but Moroni is fulfilling a scriptural expectation, and doing so by having the largest scripturally mentioned number. Therefore we see Moroni willing to have witnesses to the fullest extent possible, in at least this scriptural context. In the actual context, clearly permission was given for more, and that only amplifies Moroni’s intent to provide sufficient witness.