ANSWER: Let us look at the scriptures discussing Abraham and the meeting in question. They are found in Genesis 18, "Then the LORD appeared to him by the terebinth trees of Mamre . . . and behold, three men were standing by him . . . and bowed himself to the ground . . . So Abraham hurried into the tent to Sarah and said, 'Quickly, make ready three measures of fine meal; knead it and make cakes . . .'" (Genesis 18:1 - 2, 6).
Abraham was being hospitable to the strangers based on custom, but he may have also known one of them personally. The Wycliffe Bible Commentary states that the "father of the faithful" was an exemplary and hospitable host and that He did all he could do to serve his guests. Jamieson, Fausset, and Brown's Commentary says that a person, at that time of Abraham, who was visited by an ordinary person would stand and greet their guest. If the visitor, however, was of some 'superior rank' (or a friend), the custom was for the host to bow low then lead the visitor(s) to a tent.
The word "Lord" in Genesis 18:1 is derived from the word Yehovah, the pronunciation of which is usually Jehovah. The Bible tells us in places such as Genesis 12:1, 15:1, and so on that Abraham had seen the "Lord" several other times. In Genesis 18:3 the word "Lord" is identical to the first verse in the chapter, but is in a plural form. This likely indicates that Abraham did not know, at least initially, who his visitors really were since they looked like normal humans.
It was not until after everyone had eaten that he realized the true identity of those he served and their true purpose (see Genesis 18:9 - 15). Abraham came to understand that one of the 'men' whom he had spoken to was none other than Jesus Christ manifested as a man!
The 53rd verse of John 8 records part of a discussion between Christ and certain Jews who hated him and wanted him dead. Jesus is bluntly asked if he thinks HE is greater than Abraham and the Old Testament prophets, all of which are dead.
The amazing revelatory response of Jesus is just as blunt and to the point as the question that they asked of him. Christ told the Jews, " . . . Your father Abraham rejoiced to see My day . . ." The Jews angrily shot back their answer, 'You are not yet fifty years old . . .' Christ goes on to say that before the 'father of the faithful' was even born that HE existed as the Old Testament's I AM (verse 58)! Is it any wonder why the Jews immediately took up stones to kill him!
Genesis 18's account of the meeting between Abraham and three mysterious strangers, one of which was Jesus, testifies to how highly regarded he was in God's eyes. This is why he is called God's friend and the Eternal's friend forever in Isaiah 41:8 and 2Chronicles 20:7. The New Testament confirms his status with God when it states, "he was called the friend of God" (James 2:23).