ANSWER: Let us look at the scriptures discussing 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).
Abram was being hospitable to the strangers based on custom, but he may have also known one of them personally. The Wycliffe Bible Commentary, page 24, states regarding Genesis 18:2 that Abraham 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 regarding verse 2 that when someone at that time was visited by an ordinary person, the person considered the host would simply stand up. However, if the visitor was of some 'superior rank' (or a friend), the custom was to bow low then lead the visitor(s) to a tent.
The word "Lord" in the first verse of Genesis 18 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 the 'father of the faithful' had seen the "Lord" several other times. In verse 3 of chapter 18, the word "Lord" is identical to the first verse in the chapter, but is in a plural form. This likely indicates that Abram did not know, at least initially, who his visitors really were since they looked like any other human beings. It was not until after everyone had eaten that he realized the REAL 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! If you would like to study the other possible Old Testament appearances of Christ please see our article 'Who is the REAL Angel of the Lord?'
Genesis 18's account of Abraham's meeting with three mysterious strangers, one of which was Jesus, testifies to how highly regarded he was in God's eyes. This is why he referenced as God's friend and the Eternal's friend forever in Isaiah 41:8 and 2Chronicles 20:7. The New Testament confirms his status with the Creator when it states, "he was called the friend of God" (James 2:23).