Rashi on Genesis 18:7/רש"י על בראשית י״ח:ז׳

1בן בקר רך וטוב A CALF, TENDER AND GOOD — There were three calves so that he might give them to eat three tongues together with mustard condiment (Bava Metzia 86b).

2אל הנער TO THE YOUNG MAN — This was Ishmael whom he bade to do this in order to train him to the performance of religious duties (in this case the duty of hospitality) (Genesis Rabbah 48:13).