Q. What is the source for the seuda on the third day after the bris milah that some people do?

A. The minhag of making a seuda on the third day after the bris mila is mentioned by many Poskim. Some comment that it is not a widely kept tradition, and debate whether it is a seudas mitzva at all. See Chavas Yoir 70, Sha’arey Teshuvo (O.H. 551: 33), Sh’vus Ya’akov (3: 36), Mahari Bruno (152), et. al.. While other Poskim give it great importance. Derech Pekudecho (2: 28) writes that it is a widespread minhag and the source is the seuda that Avraham Avinu made for the visiting Malochim on the third day after his bris milah. Hanhogas Maharshal (50) maintains that this seuda is even more important than the banquet made at the bris mila itself, since it is mentioned in the Torah (beginning of Vayero). Imrei Emes (Likutim p. 96) quotes seforim that the source is from the great seuda that the Patriarch prepared on the day of “higomel” of Yitzchak ( Bereshis 21: 8). Explaining that the term stands for “on the third day after the milah.”
Horav Shlomo Miller’s Shlit’a opinion is that one should follow the traditions of his family

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a