Charles Ellicott Commentary Genesis 18:3

"and said, My lord, if now I have found favor in thy sight, pass not away, I pray thee, from thy servant:" — Genesis 18:3 (ASV)

My lord. —Hebrew ‘adonai, a term of simple respect, just as bowing towards the earth is exactly what an Arab sheikh would do even today to a passing traveler.

Abraham’s conduct is marked by all that stately courtesy usual among Eastern peoples. He calls himself their slave; regards it as a favor that they should partake of his hospitality; speaks slightingly of the meal prepared as a mere morsel of bread; and treats it as a providential act that they had come into his neighborhood.

It was only afterward that he knew he was entertaining angels unawares (Hebrews 13:2). Moreover, while he addresses the chief traveler first, as courtesy required, he immediately afterward changes to the plural, so that he would not seem lacking in hospitable welcome to his companions.