Genesis 18:2

Men in outward appearance, but angels indeed. (Hebrews xiii. 2; St. Augustine, City of God xvi. chap. 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)