Rashi writes that Avraham fulfilled the mitzvah of chinuch by giving Yishmael the job of preparing the meat for this guests who arrived (18:7). The Minchas Chinuch (264) questions whether chinuch applies only to a mitzvah chiyuvis, e.g. tefillin, lulav, which the child will inevitably be bound to perform when he becomes an adult, or does it also apply to mitzvos like aveilus, which may a person may never have to observe in his lifetime. Rav Shteinman in his Ayeles haShachar uses this aRashi as a source to prove that chinuch applies even to a non-chiyuvis mitzvah. I’m a bit surprised. Why is chessed defined as non-chiyuvis because guests and people in need don't show up every day and may not show up at all? Instead of looking at chessed as being reactive, shouldn't we be proactive and seek out opportunities to do kindness?
Another point to consider is that Rabeinu Manoch (Hil. Shvisas Asor 2:10) suggests based on the wording of the Rambam that in addition to how to perform specific mitzvos, chincuh also encompases a more general obligation to see that a child grows into a Torah observant Jew. Perhaps when Rashi refers to chinuch he means it in this global sense of inculcating Yismael with the value system which Avraham lived by.
Aside from the chinuch of Yishmael, there is an interesting Seforno that suggests that the environment of Avraham's home already impacted Yitzchak. Why did these angels need to visit when Avraham had already been told by Hashem that he would have children? The Seforno answers that they came to tell Sarah the good news so that she would also be happy and give thanks to Hashem, "k'dei she'yehiyeh ha'obar shaleim", so that the child would be "complete." It's not just the mother's mood, the feeling of simcha, which would contribute to Yitzchak's sheleimus, but it is Sarah's giving thanks to Hashem which would spiritually set the tone for his future.