http://revach.net/article.php?id=4940

Section: Parshas HaShavua   Category: Quick Vort
Parshas Mishpatim: Rav Chaim Kanievsky - Milk & Meat Are Cause For A Bris

"Lo Sivashel Gdi BaChaleiv Imo; Do not cook a goat with its mother's milk." (Mishpatim 23:19) This pasuk is written three times in the Torah. The Mechilta says that this is K'neged the three times Hashem entered into a Bris with Bnei Yisrael: once at Har Sinai, once in Arvos Moav, and once at Har Grizim. Why is the issur of Basar V'Chalav tied to our Bris with Hashem?

Rav Chaim Kanievsky answers that the Pesikta Rabasi says that the Malachim asked Hashem to give them the Torah. Hashem said to them that in the Torah it is written do eat meat and milk together, yet at Avrohom's house when the three Malachim came to visit him, they ate milk and meat. The Malachim could not answer this charge so Hashem took the opportunity to tell Moshe to write the Torah for Bnei Yisrael, and He entered into a Bris with us.

It was with the issur of basar b'chalav that Hashem gave us the Torah and entered into a Bris.  Therefore, the issur is written three times kneged the three brissim.