Why does there have to be three in the Godhead or why is there three in the Godhead?
By having three members in the Godhead, we find God exemplifying the law of witnesses that he gave to His children (see also D&C 6:28 and 128:3). Jesus Himself followed this law during His mortal sojourn. Addressing the Pharisees, Jesus taught that the law of witnesses worked in His favor. “It is also written in your law that the testimony of two men is true. I am one that bear witness of myself, and the Father that sent me beareth witness of me” (John 8:17-18). Although the Pharisees did not recognize this second witness, it was nonetheless there (as testified by Peter (Matt. 16:25-17)).
At another time, Jesus again taught this principle and listed off His witnesses (John 5:32-37). “There is another that beareth witness of me; … Ye sent unto John, and he bare witness unto the truth. But I receive not testimony from man: but these things I say, that ye might be saved.” Ultimately the Father is the best witness for Christ, but since all do not have the faith of Peter, other witnesses are provided that are more easily recognized by the spiritually dense. The Lord continues, “but I have greater witness than that of John: for the works which the Father hath given me to finish, the same works that I do, bear witness of me, that the Father hath sent me.” The second witness is His actions. It is the same witness that his disciples should have testifying that they are indeed following their Master (Matt. 7:16-20). And just to round out the list, Jesus offers a third witness: “And the Father himself, which hath sent me, hath borne witness of me.” The testimony of these three witnesses ensure that the judgments of God against the hardened Pharisees are just.
At the Last Supper, Jesus again draws on this principle while introducing the Comforter (or Holy Ghost) to His chosen apostles. “But when the Comforter is come, … he shall testify of me”. And that’s not all. The apostles themselves are to testify as well: “And ye also shall bear witness, because ye have been with me from the beginning” (John 15:26-27). This tradition carries on today, when we have modern apostles testifying publicly of the works Christ continues, and the Holy Ghost confirming their words as a second witness. The next time you attend General Conference, listen for this testimony from modern witnesses.
Jesus followed the Last Supper with a prayer that provides us with another lesson we can learn from the Godhead. Picking up on themes He had just taught the apostles, Jesus prayed “that they may be one, even as we are one: I in them, and thou in me, that they may be made perfect in one” (John 17:22-23). That ancient governing body of the primitive church were to pattern themselves after the presiding heavenly council in unity, love, and perfection. Our modern councils should do the same. And given that there are three in the Godhead presidency, we find a special emphasis on mortal presidencies following suit as well.