28 Lest you fall into the error of supposing that this acknowledgment of the One was a payment of honor to all the three whom Abraham saw in company, mark the words of Lot when he saw the two who had departed; And when Lot saw them, he rose up to meet them, and he bowed himself with his face toward the ground; and he said, Behold, my lords, turn in to your servant's house. Here the plural lords shows that this was nothing more than a vision of angels; in the other case the faithful patriarch pays the honour due to One only.
Thus the sacred narrative makes it clear that two of the three were mere angels; it had previously proclaimed the One as Lord and God by the words, And the Lord said unto Abraham, Wherefore did Sarah laugh, saying, Shall I then bear a child? But I am grown old. Is anything from God impossible? At this season I will return to you hereafter, and Sarah shall have a son. The Scripture is accurate and consistent; we detect no such confusion as the plural used of the One God and Lord, no Divine honours paid to the two angels. Lot, no doubt, calls them lords, while the Scripture calls them angels. The one is human reverence, the other literal truth.
Source: On the Trinity (New Advent)