Genesis 18:1-2 New International Version (NIV)
The Three Visitors
18 The Lord appeared to Abraham near the great trees of Mamre while he was sitting at the entrance to his tent in the heat of the day. 2 Abraham looked up and saw three men standing nearby. When he saw them, he hurried from the entrance of his tent to meet them and bowed low to the ground.
Ellicott’s Commentary for English Readers
The number three pointed also to the Trinity of Persons in the Godhead, and is therefore read by our Church as one of the lessons for Trinity Sunday. But we must be careful not to use it as a proof of this doctrine, lest the inference should be drawn of a personal appearance of the Father and of the Holy Ghost, which would savour of heretical impiety.
Gill’s Exposition of the Entire Bible
,”the three Persons in the Trinity, in the shape of three men, appear to Abraham and dine with him, and eat the first flesh mentioned eaten in all the Scripture.’But to this may be objected, that the Father and the Holy Spirit are never said to appear in an human form, see John 5:37; or are ever called angels, as these are, Genesis 19:1; but they rather seem to be angels, as the Targums of Jonathan and Jerusalem call them, in the likeness of men, who were sent on three messages, as they suggest; one to bring the news of Sarah’s bearing Isaac; the other to deliver Lot; and the third to overthrow Sodom and Gomorrah; which is a much better sense than that of Ben Gersom, who takes them to be three prophets, and mentions the names of two of them, Shem and Heber; for two of these are expressly called angels, Genesis 19:1; and the apostle seems to refer to this history, Hebrews 13:2; in the Talmud they are said to be Michael, Gabriel, and Raphael
Matthew Poole’s English Annotations on the Holy Bible
Three men, as they seemed to be, though indeed they were angels in men’s shape.