Genesis 18:1-5 (ESV)
1 And the LORD appeared to [Abraham] by the oaks of Mamre, as he sat at the door of his tent in the heat of the day. 
2 He lifted up his eyes and looked, and behold, three men were standing in front of him. When he saw them, he ran from the tent door to meet them and bowed himself to the earth
3 and said, “O Lord, if I have found favor in your sight, do not pass by your servant.
4 Let a little water be brought, and wash your feet, and rest yourselves under the tree,
5 while I bring a morsel of bread, that you may refresh yourselves, and after that you may pass on- since you have come to your servant.” So they said, “Do as you have said.”
Who are these “three men” from verse two who were standing in front of Abraham? We know that at a minimum in some way these three men represented the LORD to Abraham since verse 1 tells us that “the LORD appeared to” Abraham. These “men” weren’t human men as the text of Genesis 18-19 make clear. They were heavenly beings.
But could one of these men have actually been “the LORD”? John Sailhamer argues that this could raise difficult questions since the Pentateuch specifically forbids any presentation of God in any physical form.
Moses spoke to God’s people in Deuteronomy 4:15, “Therefore watch yourselves very carefully. Since you saw no form on the day that the LORD spoke to you at Horeb out of the midst of the fire.”
This, of course, is a reference against idolatry, but does this mean that God would never appear in a physical form before man? The incarnation of the Christ in Jesus of Nazareth would appear to argue against that conclusion.
The second commandment (see Exodus 20:4) prohibits human beings from making an image of God. It doesn’t prohibit God from showing up in physical image. But, then again, God did tell Moses that no man could see God and live (see Exodus 33:20).
This, of course, brings us back to the incarnation. We believe that Jesus of Nazareth was the Christ, the Son of God, the second person of the Trinity, and we believe that he was God in the flesh (see John 1:1, 14; Colossians 1:15-20; Hebrews 1:1-4; etc.). Jesus of Nazareth was clearly seen by others, even though he was (and is!) God.
Bruce Waltke argues that the three men of Genesis 18 are “actually the Lord and two angels” (page 266). He continues, “The later identifications of the ‘men’ (18:10, 13, 16-17, 33; 19:1) confirm their manifest difference. One man is none other than the Lord, as 18:2-3 and especially 10, 13-15 make explicit” (266-67).
This “Lord” would be none other than the pre-incarnate Christ, the second person of the Trinity.
For His Glory,
Pastor Brian