This is from the Aquinas Study Bible
18:1-2 St. Justin Martyr (Dia Try), Origen (Hom Gen) and Theodoret of Cyrus (Com Heb) believed the three to be two angels and the Lord Jesus, while St. Augustine (Cit God) believed them to all be angels. Philo of Alexandria (On Abr 22.113) assumed that “Abraham imagined them to be either some prophets or angels who had changed their spiritual and soul-like essence, and assumed the appearance of men.”
Abraham adored them, thinking that they were holy men sent from God. But he adored them with adoration called dulia, which is shown to saints; and he offered them food, as though they were men. (St. Thomas Aquinas Com Heb 13:2)
18:9 when they had eaten: The food taken by angels was neither changed into the assumed body, nor was the body of such a nature that food could be changed into it; consequently, it was not a true eating, but figurative of spiritual eating. This is what the angel said to Tobias: “When I was with you, I seemed indeed to eat and to drink; but I use an invisible meat and drink” (Tobit 12:19). (St. Thomas Aquinas Sum Theo 51.3)