Why did Avraham run to the bulls?
Seforno: Refer to 18:5:3:3*.
Pirkei d'R. Eliezer, Hadar Zekenim: When Avraham approached the bull to slaughter it, it fled, and Avraham gave chase. The bull actually led him to the Me'aras ha'Machpelah. 1
Ramban (to 18:8): So great was Avraham's desire to perform kindness that, in spite of the three hundred and eighteen able-bodied members of his household, his advanced age and his current physical condition, he opted to go personally to Sarah's tent to encourage her to quickly prepare bread, and then ran to the meadow to choose a juicy bull for his guests!
That is how Avraham discovered the Me'aras ha'Machpelah. (See also Ba'al ha'Turim).
QUESTIONS ON RASHI
Rashi writes: "'A bull, tender, and good' - these [total] three bulls." But perhaps it was only one bull, described in these two ways?
Gur Aryeh (citing Bava Metzia 86b): The extra term 'and' shows that the 'good' one was a distinct item; and if so, the 'tender' one was distinct as well.
Rashi writes: "These [total] three bulls." Why doesn't the verse write this briefly and explicitly?
Gur Aryeh: The Torah would then have had to explain why he needed three whole bulls; 1 so it prefers to leave this as a hint.
Rashi: He wished to serve tongue in mustard to each of his three guests. Refer to 18:7:1.3:1.
Rashi writes: "Three tongues in mustard." How can we possibly know this?
Gur Aryeh: Tongue in mustard is a nobleman's delicacy, 1 and Avraham needed whole tongues, for one serves honored guests an entire cut of meat. 2 Thus, if Avraham slaughtered three whole animals, it must have been to serve them exactly this dish. 3
Tongue is part of the gifts to Kohanim from slaughtered animals, and they are told to eat it "in nobility" - meaning with mustard (Chulin 132b).
Known in Halachah as a Chatichah ha'Re'uyah l'Hiskabed (Chulin 100a).
Other cuts of meat come in pairs, one from either side of the animal, and Avraham would have needed two animals at most. Only the tongue is located in the middle, requiring three animals. (CS)
Rashi writes: "'To the youth' - This was Yishmael." How do we know this?
Gur Aryeh: The Torah writes "to the youth" with the definite article; it must refer to Yishmael.
Rashi writes: "To train him in performing Mitzvos." How do we know this was the reason?
Gur Aryeh: Otherwise, why delegate to Yishmael specifically?