|BE STILL AND LISTEN
Peter was often quick with the lip but slow with the wit. When Jesus, the Son of the One True Living God was transfigured into His eternal glory, it wasn't time for chit chat. There was no need to fill up the moment with nervous chatter.
The transfiguration of Christ was shared with only the inner circle of disciples (Peter, James, and John). God doesnt reveal His glory to just anyone. And for Moses and Elijah to appear was a mind blowing experience. (Especially when you consider the mystery surrounding the death of both but dont get me started.)
This event was not meant to blow their minds as much as to clarify in their minds once for all the reality of Jesus as not of this world, (John 8:23, 18:36) as the One with the keys of hell and death (Revelation 1:18), and as God the Beloved Son, in whom God the Father was well pleased.
The Father rarely speaks audibly but Peter was ruining this important moment. So He stepped in to shut Peter up. Peter needed to be quiet, pay attention, and take it all in. Thats what James and John were doing.