He took cream and milk and the calves that he had prepared, and he placed [them] in front of [the angels], and he was standing over them under the tree, and they ate.

-- Breshis 18:8

Classic Questions

How could Avraham serve milk and meat together? (v. 8)

Da'as Zekeinim: Rashi writes that Avraham served them "bit by bit, as each item was prepared" (v. 8). Thus, he did not serve them meat and milk together, which is forbidden, but rather milk, and then afterwards meat ("cream and milk, and the calves"), which is permissible.

The Rebbe's Teachings

Milk & Meat (v. 8)

In addition to the argument of Da'as Zekeinim that the cream and calves were served separately, thus circumventing the prohibition of eating milk and meat, one could also argue that he offered each of the angels either milk or meat. Thus, "three calves' tongues with mustard" were prepared (Rashi, v. 7) to give each of the guests the option of eating meat. This was done in order to fulfill the mitzvah of welcoming guests in the best possible fashion. No guest, however, was served milk and meat.

(Based on Likutei Sichos vol. 6, p. 150)

See also Did Abraham Serve His Guests Non-Kosher?