By the Words of Three I Will Establish My Word

After using the Gold Plates to record Jacob’s, Isaiah’s, and his own testimony of Christ’s gospel, Nephi states the following:

“…By the words of three, God hath said, I will establish my word. Nevertheless, God sendeth more witnesses, and he proveth all his words.” (2 Nephi 11:3)

Questions:

-Is this principle of three witnesses found in other places in the scriptures?

-Thinking of the Godhead as an example, are the testimonies given by the Father, the Son, and the Holy Ghost from a single source? Or, are these testimonies from three separate Persons who are one in purpose?

-Does each member of the Godhead share His testimony in a unique way? Where in the scriptures do we have the testimony of the Father? Where in the scriptures do we have the testimony of the Son? How does the Holy Ghost share His testimony?

-Did God establish three witnesses to the divine origins of the Book of Mormon? Where can the testimony of those three witnesses by found? (see Testimony of the Three Witnesses)