The number of witnesses conforms to the instructions in Deuteronomy 17:6: “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.” This law is spelled out for capital cases—those involving adjudications of life and death. This law of witnesses is again invoked for the coming forth of God’s work. Even though two witnesses are the minimum, three is better, implying an even more solid establishment of truth.
The later calling of an additional eight witnesses shows the difference between the invocation of the scriptural law (three) and the reality, in which an additional number (eight) are called. The fact of the eight actually reinforces Moroni’s intention of providing a maximal number to be sufficient. It may not be a coincidence that, including Joseph Smith, twelve men testified to the plates’ reality.