What happened to the bread that Sarah prepared?
Rashbam: It is so obvious that Avraham served it, that the Torah does not find it necessary to mention it.
Sifsei Chachamim - her limbs felt heavy, and she separated from the dough lest she be Metamei it, and before she found someone else to finish it, it became Chametz 3 , and it was Pesach.
If so, why did she laugh at the idea that her Ednah (menstrual cycle, according to Rashi's second Perush) would return? Tosfos (Rosh Hashanah 11b) - she thought that it was a fluke that she became Nidah, but not that her cycle was returning.
Bartenura - he would not seve to his guests something that he himself would not eat. Chasam Sofer (Pesachim 46b) - it was Yom Tov of Pesach. Yisre'elim may not bake on Yom Tov just for Nochrim if they themselves will not eat.
It is difficult to say that this is Rashi's intention, that "Tamei" means forbidden due to Chametz, amidst concern for Tum'ah (PF).
Why (according to Rashi) did Avraham see fit to prepare three bulls for his visitors?
Rashi (citing Bava Metzi'a 86b): In order to serve each one of them a tongue (the tastiest part of the animal). 1
See Sifsei Chachamim for Rashi's source. According to Targum Yonasan and the Ramban there seems to have been only one bull.
How could Avraham, who kept the entire Torah, serve his guests meat and milk during the same meal?
Rashi: He served the items in the order that he prepared them (as they appear in the Pasuk) - first milk and then meat. 1
According to the Gemara in Shabbos, it was precisely because the angels ate Basar be'Chalav that Moshe was able to convince them later that the Torah is meant for Yisrael, and not for the angels.
Since when do angels eat?
Rashi, Da'as Zekenim and Targum Yonasan: They pretended to eat, because one should not change from the local custom. 1
The Gemara in Bava Metzi'a (86b) explains that, by the same token, when Moshe ascended to the Heaven (at Matan Torah), he did not eat for forty days.
Under which tree did the angels rest?
Rashbam: Under one of the trees in the orchard.
What does the Torah mean when it states "he was standing over them under the tree"?
Rashi (in Chayei Sarah, 28:30) and Targum Yonasan: He stood over them to serve them (whilst they sat underneath the tree
What is the meaning of the word "the bull that he made"?
Da'as Zekenim: It means that he prepared it.