Genesis 18:8

ויקח חמאה וגו AND HE TOOK חמאה, ETC. — but he did not bring bread, on account of Sarah becoming mentstruant; as the 'way of women' returned to her that day, and the dough became impure (Bava Metzia 87a).

and he took rich cream and milk and the calf which the young man had made into prepared meats, and set them before them, according to the way and conduct (hilkath) of the creatures of the world; and he served before them, and they sat under the tree; and he quieted himself (to see) whether they would eat.

ויקח חמאה וחלב, according to the plain meaning of the text there was no need to report that Avraham supplied the bread which he had volunteered to serve these men. It was only necessary to list the additional items he served, seeing no mention had been made of them thus far.

עומד עליהם, when a person is seated and someone else stands beside him, the one standing is referred to with the preposition עליו. Compare Isaiah 6,1-2 ראיתי את ה' יושב על כסא ושרפים עומדים ממעל לו. “I saw G’d seated on a throne and the Seraphim were standing beside Him.” [it is hardly likely that Isaiah in this vision saw the angels as standing above G’d. Ed.] A similar construction is found in Kings I 22,19 involving a vision by Michyahu in response to Yehoshaphat’s request.

תחת העץ, a reference to trees, as in an orchard.

ויקח, he, personally.

ויתן, he placed before them butter and milk as well as the young calf. He offered them the choice to eat either dairy or meaty. There was no need to mention the bread as it is understood that he first brought them the bread. After all, the very minimum one can serve guests is bread. When one entertains guests who appear to be distinguished people one most certainly does not fail to offer them bread first. Besides, the Torah had already mentioned that he prepared the bread in the guise of uggot, cakes. There was also no need to mention that he served them wine, another, staple at all meals.

אשר עשה, by means of his servant.

והוא עומד, as explained by Onkelos, i.e. he served them in the capacity of waiter.

עליהם, the word is used in the same sense already in verse 2

ויאכלו, this is how it appeared to him. All this was part of his prophetic vision.

ויקח חמאה וחלב, “he took curd and milk, etc.” this teaches that he fed them both milky and meaty dishes. When G–d was ready to give the Torah to the Israelites, the angels are reported as having said to Him (Psalms 8,2): תנה הודך על שמים, “You have covered the heavens with Your glory” [This is perceived as the angels demanding that the Torah remain in the celestial regions of the universe. Ed.] G–d replied to them: “in My Torah it is written that milky and meaty dishes not be consumed at the same meal (Exodus 23,19), yet when you descended to earth you partook of such a mixture at Avraham’s table. (The inference is clear, i.e. “the laws of the Torah do not apply to you.”) As soon as the angels heard this they withdrew their objections to the Torah being given to people on earth. This is the deeper meaning of Exodus 34,27: כי על פי הדברים האלה כרתי אתך ברית ואת ישראל, “for in accordance with these words I have made a covenant with you and with Israel.” What do “these words” refer to? They refer to the verse prior to this where eating meaty and milky dishes at the same time has been prohibited. Nonetheless, this metaphorical explanation appears to contradict another metaphorical interpretation (Midrash) in which we have been taught that Avraham personally observed not only all the Biblical laws of the Torah, but even the “fences” surrounding them to protect us against violating the prohibition itself. The particular example quoted there for such a “fence” is interestingly the subject of eyruv tavshilin, certain preparations of food when the holiday occurs immediately before the Sabbath. But that is not the only way of explaining the Midrash. The word: eyruv, actually means “mixture,” so that the Midrash may have referred to a mixture of meaty and milky courses of the same meal, so that there is no contradiction at all. Seeing that Avraham is reported as first having served milky dishes, and the meat subsequently, it is reasonable to assume that he wished to give his guests something to eat before freshly slaughtered animals, that had to have their blood removed could be cooked or roasted on the spit. The interval would have sufficed not to violate the laws of mixing milk and meat. [If people ate their main meal during the fourth hour after sunrise, seeing that these men (angels) did not arrive at Sodom until evening, there would have been plenty of time for their eating both. Ed.]