Mamre

This was the name of the man who owned the oak trees.

in the tent doorway

"at the opening of the tent" or "at the entrance of the tent"

"hottest time of day"

"He looked up and saw, and behold, three men were standing"

"suddenly." The word "behold" here shows us that what follows was surprising to Abraham.

"nearby" or "there." They were near him, but far enough away for him to run to them.

This means to bend over to humbly express respect and honor toward someone.