Promise of Three Witnesses

In the Doctrine and Covenants Joseph Smith made public to the world that their would be Three Witnesses to the Book of Mormon. This promise is found in D&C 5:11-13 where we read:

11 And in addition to your testimony, the testimony of three of my servants, whom I shall call and ordain, unto whom I will show these things, and they shall go forth with my words that are given through you.
12 Yea, they shall know of a asurety that these things are true, for from heaven will I declare it unto them.
13 I will give them power that they may behold and view these things as they are;

Oliver Cowdery, David Whitmer, and Martin Harris did later see the gold plates and hear the voice of God declare unto them that the translation made therof was by the gift and power of God. That is an amazing prediction for an imposter or fraud to make. However when a prophet does it we call it prophecy.