John Calvin Commentary Genesis 18:9

"And they said unto him, Where is Sarah thy wife? And he said, Behold, in the tent." — Genesis 18:9 (ASV)

Where is Sarah? Until now, God permitted Abraham to discharge an obvious duty. But, having given him the opportunity of exercising charity, God now begins to manifest himself in his angels. The reason why Moses introduces, at one time, three speakers, while, at another, he ascribes speech to one only, is that the three together represent the person of one God. We must also remember what I have recently mentioned, that the principal place is given to one; because Christ, who is the living image of the Father, often appeared to the fathers under the form of an angel, while, at the same time, he also had angels, of whom he was the Head, as his attendants. And as to their making inquiry about Sarah; we may therefore infer that a son is again promised to Abraham here, because she had not been present at the former oracle.