It seems that Jesus went up the mountain with Peter, James and John, not primarily in order to manifest his glory to them but simply in order to pray. It was his deep prayer experience which transfigured him and made him shine with God's glory.

This is a model for what Christian prayer should eventually be: a transfiguration of our lives through prayer so that we are filled with the glory or at least the grace of God. In prayer, let us ascend the mountain with Jesus: let us ask for the grace of transforming prayer, prayer which transfigures our lives.

When Peter offers to build three booths [tents], there is a certain amount of goodness in his suggestion, since the three tents are not for himself, James and John, but for Jesus, Moses and Elijah. However Peter's intention or suggestion was misguided. - Not all good ideas or good intentions are worth following up. Hence the voice of God the Father from heaven calls Peter back to reality: "This is my Beloved Son. Listen to him" - as if he were saying: set aside your plans and give your attention to Jesus. We often have to let go of our own good ideas to listen to what Jesus is asking of us.