Gen. 18:21 "I will go down to see ..." - This seems to be a very physical task. Why would HaShem have to go down and see? Doesn't he already know?
Perhaps this is HaShem speaking through one of the angles. Those angles were in fact looking down at Sodom, Gen. 18:16. And they would be soon departing to ~go down~ to Sodom. It's fitting then that HaShem would, acting through the angles, go down and see.
Doesn't it then make sense that after the angles revealed to Abraham that Sarai would bear a son (Only HaShem had told him of this) that Abraham knew then, that they were angles of HaShem. Meaning right after that revelation, when they talked they were HaShem (since Abraham now knew it, and the Torah seems to let us know only what is apparent to Abraham), but when they acted they were men carrying out G-d's commands.
This is why it says "... Avraham still stood in the presence of YHWH." Everett FOX Gen. 18:22. Abraham stood in the presence of the one angle who stayed - had completed his job. The other two went on to Sodom.
If you consider this encounter of Abraham with HaShem Gen. 18:13-33, that HaShem was speaking through the angel that was with Abraham, then the whole encounter makes more sense.
PS. It shows Abrahams character that even before he knew that these men were angles, he still treated them with such hospitality.