Did Avraham serve his visitors a non-kosher meal in Genesis 18:8, "And he took butter, and milk, and the calf which he had dressed, and set it before them; and he stood by them under the tree, and they did eat."?

Our sages teach us that Avraham had the order in a specific way – first milk and butter – then meat. That is the Halacha as well – you can have meat after milk but not the other way around.