‘And they said to him, “Where is Sarah, your wife?” And he said, “Why, in the tent.”
The question is still from the impersonal ‘they'. Abraham knows they are important but is not yet aware of the One with Whom he is dealing.
‘And he said, “I will certainly return to you when the season comes round (or when the spring comes), and lo, Sarah your wife will have a son”.'
This is the moment when the leader of the three reveals Himself as a special messenger of Yahweh. The ‘they' becomes ‘he', and the promise of a son through Sarah is renewed. (‘When the season comes round' may mean ‘when the conception matures into birth', indicating that the child is already conceived).
Genesis 18:10-11 (18:10b-11)
‘And Sarah overheard in the tent door which was behind him. Now Abraham and Sarah were old, and well aged. It had ceased to be with Sarah after the manner of women.'
Sarah was possibly hidden behind the tent door listening in to what was said, or alternatively is standing in the tent door, visible but discreet, ready to watch over any further needs of the visitors. The writer then makes clear that nature had caught up with Sarah. Her periods had ceased. The birth of a son was seemingly impossible.