Shalom, In Bereshit 18:8, It says that Avraham gave milk and meat to visitors, there are Midrash that tells us that Avraham compressed the laws, and how he can do that, is not a contradiction with the Halakha?
Your question is discussed by the Midrash, and the Rishonim, and there are a number of answers to this. See Daas Zekenim, who gives two answers. One Midrash says that when H-shem wanted to give the Torah and the Malachim objected that how can you give the torah to mere mortals, keep the torah by us. To this H-shem answered them, “When you were put into the nisayon you did eat milk and meat”, therefore you can’t complain that the Torah is bring given to man. When the Malachim heard this they agreed that we can get the Torah. He brings another Midrash that says differently, that Avrohom in fact didn’t do anything wrong, because he gave them butter first and then the meat, and it is permitted to eat meat after milk. In fact this is why Rashi explains that he brought each item to them separately.