Ambrose of Milan

Hospitality is good, it has its reward, first the gratitude of man, and then, what is greater, the reward of God. We are all guests in this abode; for a time we have a place to stay: we are emigrating soon. Let us beware that if we have been harsh or negligent in receiving guests, even the holy hostels may be denied to us after the course of this life. Hence in the Gospel the Savior says: Make friends for yourselves with unjust wealth, so that they may receive you into eternal dwellings. …

On Abraham ↗

Bede

And when he had lifted up his eyes, he saw three men standing near him; and when he saw them, he ran to meet them from the door of the tent, and worshipped upon the ground, and said: Lord, if I have found grace in thy sight, do not pass by thy servant, etc. …

Commentary on Genesis (Hexaemeron) ↗

Bonaventure

For testimony is given in the Old Testament concerning the Trinity both in figures and in words. Among the figures, however, that one was especially authoritative which was shown to the father of our faith, Abraham: of whom it is said in Genesis eighteen, that when three men appeared to him, he indeed saw three, but adored one, whence he spoke to the three as to one, in this understanding in himself and intimating to others the mystery of the Trinity.

Disputed Questions on the Mystery of the Trinity, Question 1 ↗