Ambrose of Milan — as excerpted in the Catena Aurea on Luke 9:28-31

“I should think that in the three who are taken up into the mountain, was contained in a mystery the human race, because from the three sons of Noah sprung the whole race of man; I did not perceive that they were chosen out. Three then are chosen to ascend the mountain, because none can see the glory of the resurrection, but they who have preserved the mystery of the Trinity with inviolable purity of faith.”