Genesis 18:1 KJV [1] And the LORD appeared unto him in the plains of Mamre: and he sat in the tent door in the heat of the day; https://bible.com/bible/1/gen.18.1.KJV
And the lord appeared same as chapter 17 vs 1 “And when Abram was ninety years old and nine, the LORD appeared to Abram, and said unto him, I am the Almighty God; walk before me, and be thou perfect…” 👌
However here we see that God’s appearing was in physical form. I believe the previous appearances were also physical except otherwise stated by the text.
Genesis 18:2-3 KJV [2] 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, [3] and said, My Lord, if now I have found favour in thy sight, pass not away, I pray thee, from thy servant: https://bible.com/bible/1/gen.18.2-3.KJV
How did he know who they were? Could it be the suddenness of their appearing.
Or could it be that he recognized one of them from previous God encounters? One of which was not too long ago. Barely a year actually because by 100 years of age, Isaac was born and at 99 God had appeared to Abraham so this 2nd appearing was between 99 years and the 9 months preceding Isaac’s birth.
Even within the sudden appearance of these men there might have been a semblance to how God had been appearing to him in time past. This MOG ABRAHAM knew God and could therefore tell this was God