2 Nephi 11

Nephi comments of Isaiah that -

"...he verily saw my Redeemer, even as I have seen him."  (Book of Mormon | 2 Nephi 11:2)

Then adding -

"...my brother, Jacob, also has seen him as I have seen him..."  (Book of Mormon | 2 Nephi 11:3)

Now with these three testimonies, Nephi writes -

"...wherefore, I will send their words forth unto my children to prove unto them that my words are true. Wherefore, by the words of three, God hath said, I will establish my word. Nevertheless, God sendeth more witnesses, and he proveth all his words.
4 Behold, my soul delighteth in proving unto my people the truth of the coming of Christ..."  (Book of Mormon | 2 Nephi 11:3 - 4)

I find it interesting that the Lord said that three witnesses is all that is necessary to establish or prove His word...even though He sends out many more witnesses.  Therefore, we will stand accountable with only three witnesses...it is proof enough.