3 Peter sees this, and as a man savouring the things of men says, “Lord, it is good for us to be here.” He had been wearied with the multitude, he had found now the mountain's solitude; there he had Christ the Bread of the soul. What! should he depart thence again to travail and pains, possessed of a holy love to Godward, and thereby of a good conversation? He wished well for himself; and so he added, “If You will, let us make here three tabernacles; one for You, and one for Moses, and one for Elias.” To this the Lord made no answer; but notwithstanding Peter was answered. “For while he yet spoke, a bright cloud came, and overshadowed them.” He desired three tabernacles; the heavenly answer showed him that we have One, which human judgment desired to divide. Christ, the Word of God, the Word of God in the Law, the Word in the Prophets. Why, Peter, do you seek to divide them? It were more fitting for you to join them. You seek three; understand that they are but One.
Source: Sermons on Selected Lessons of the New Testament (New Advent)