John Calvin Commentary Genesis 18:5

"and I will fetch a morsel of bread, and strengthen ye your heart; after that ye shall pass on: forasmuch as ye are come to your servant. And they said, So do, as thou hast said." — Genesis 18:5 (ASV)

For therefore are ye come to your servant. He does not mean that they had come intentionally, or for the express purpose of seeking to be entertained as his guests. Instead, he suggests that their arrival occurred at a fortunate time, as if he would say, ‘You have not slipped into this place by chance, but have been led here by the design and direction of God.’ Therefore, he attributes it to God's providence that they had arrived so conveniently at a place where they could refresh themselves for a little while, until the heat of the sun subsided.

Moreover, since it is certain that Abraham spoke this way in sincerity of mind, let us, following his example, conclude that whenever our brothers and sisters who need our help meet us, they are sent to us by God.