I’ve seen two opinions (worth mentioning) regarding this story. One says that all three “men” were angels, and the other says that one of them was the Son of God.
Here are some quotes I found:
“For in defence of feasts your Fraternity mentions the feast of Abraham, in which by the testimony of Holy Scripture he is said to have entertained three angels.” Pope St. Gregory the Great
*“No doubt the Almighty Son of God could have appeared for the purpose of teaching, and justifying men in exactly the same way that He appeared both to patriarchs and prophets in the semblance of flesh ; for instance, when He engaged in a struggle, and entered into conversation (with Jacob), or when He refused not hospitable entertainment, and even partook of the food set before Him.” * Pope St. Leo the Great
Notice St. Leo says “the semblance of flesh”, meaning the Son only took on the outward appearance of a man, but was not truly a man because He had not been born of the Virgin Mary yet.
“Men in outward appearance, but angels indeed. Heb. xiii. 2. S. Aug. de C. D. xvi. c. 29. Some have supposed, that one of them was the Son of God, whom Abraham adored, and who bears throughout the chief authority. Tres vidit et unum adoravit. He saw three and adored one, as we read in the Church office. In the former supposition, which is generally adopted, this adoration was only a civil ceremony, if Abraham considered them as mere men; or it might be mixed with a degree of religious, though inferior veneration, if he imagined they were angels; or in fine, he adored God in his representatives.” Haydock Commentary
Notice the reference to Hebrews 13:2 in the Haydock quote:
“And hospitality do not forget; for by this some, being not aware of it, have entertained angels.”
“They imagined they received men, when they were Angels. The Latin interpreter followed exactly the Greek, though the expression be unusual in both languages. It is meant of Abraham, (Gen. xviii. 2.) and of Lot, (Gen. xix. 1).” Haydock Commentary