I was learning that the "angel of the LORD" in the Old Testament, or otherwise known as "Ha Shem, The Name" was understood by the early Fathers to be the person of Jesus in his divine nature before he incarnated. Perhaps this is partially where Jehovah's Witnesses get him mixed up with St. Michael the Archangel?
So when the "angel of the LORD" appeared to Abraham at Mamre, with the other two -- that was a foreshadowing of the Trinity. While yes, two of the angels.. were angels to carry out the judgement upon Sodom and Gomorrah only one of them was Christ as God the Son. There are hints and glimpses of Christ and the Church throughout Moses, and the Prophets, and the Writings. We considered them to be "types" of that which is to come, as that's how we understand Jesus to have been mentioned in the Scriptures when our Jewish friends will never accept that.