Denying the visitor's lordship in Gen. 18 is denying God. Adonai is a common title of God, and it makes perfect sense to have the Lord addressed in this title since the name YHWH had NOT been revealed yet. I don't care what kind of translation you read, in KJV and NKJV, Abraham was talking with the LORD, in all caps, maybe not in 18:3, but in all the rest of the chapter, that refers to nobody else but God Almighty, the Creator of the universe.
Okay let's look at the KJV in a different spot where the word adonai is used. Let's look at Ezra 10:3 and verse 8. The counsel of the "lord" (adonai) in Ezra 10:3 is in reference to humans in the context. That isn't my translation. Did Ezra deny God, too? Why can't adonai not be God when there are examples of adonai not referring to God? That isn't a denial. Genesis 18 does say that they are "men" who appeared after all.
Ezra 10
3Now therefore let us make a covenant with our God to put away all the wives, and such as are born of them, according to the counsel of my lord, and of those that tremble at the commandment of our God; and let it be done according to the law.
8And that whosoever would not come within three days, according to the counsel of the princes and the elders, all his substance should be forfeited, and himself separated from the congregation of those that had been carried away.