There are a number of possibilities:
- Angels, acting as visible representatives of God, appeared in the name of the Lord. These angels speak for the Lord, so the Lord is speaking through them.
- Jesus showed up with two angels.
- All three members of the Godhead appeared in physical form.
In Genesis 18:22, the passage says two men (angels as we see in Genesis 19) walked toward Sodom while Abraham stood with the Lord. So if at least two of three are angels, the third would probably be either Jesus, who sent the angels to Sodom, or another angel who, for some reason, stayed behind. Most scholars believe it was Jesus since the book of Hebrews makes the case that Jesus was the priest Melchizedek. It's possible then that he could appear again.