Although verse 2 of this chapter allows for the eight witnesses, the instructions are specifically for the three witnesses. Those were the first to receive that privilege. The reason for choosing three is based on the Deuteronomic requirement of the number of witnesses that would establish truth. Therefore, the three are intended to represent a truthful witness.
Verse 4 indicates that they would not only be able to testify of the plates, but also of the “power of God.” Thus, it was only the three that received the visionary experience.
The promise to the three is that if they are repentant and continue in faithfulness, they would “come unto the Father in the name of Jesus, [and] they shall be received into the kingdom of God.” Even though the three may have wavered in their testimony of Joseph, they never did waver in their testimony of the plates and their experience as witnesses.