And the LORD appeared unto him in the plains of Mamre: and he sat in the tent door in the heat of the day; And he lift up his eyes and looked, and, lo, three men stood by him: and when he saw them, he ran to meet them from the tent door, and bowed himself toward the ground, Genesis 18:1-2
This is a very interesting text, here we see Abraham says the Lord “Yahweh” appears to him. The original Hebrew in the text has YHWH translated as LORD in English. So we know that Abraham is saying God appeared to him
The next part is astonishing as God appeared to Abraham in the form of 3 men. People who say the Triunity of God was made up by the Romans or Greek mythology has never studied this text. Here we see the Triunity of God in the Torah.
This is symbolic of the Father, Son , and Holy Spirit. So we can make assumption that one of these men were Jesus Christ. Abraham has now met Jesus face to face and the text says Abraham bowed to the ground to them
And he said, I will certainly return unto thee according to the time of life; and, lo, Sarah thy wife shall have a son. And Sarah heard it in the tent door, which was behind him. Genesis 18:10
Why did God appear to Abraham in human form? Well first remember the seed of the Messiah was going to come from Israel by Abraham. So Abraham had divine revelation from God more than anyone at this point.
Second Jesus had a special prophetic message to deliver to Abraham. That by next year the promised Son of Isaac would be here. This is the blessing that Abraham and Sarah have been waiting for years!