My 3 year old son refuses to eat most the time he drinks a lot of milk anywhere from 2/6 cups a day easily drinking a gallon in two days is this bad?

Yes, it's bad: That is not an appropriate diet for a 3year old. He is not getting many of his required nutrients, vitamins and minerals with that much milk. First, solve milk problem. Once he's not getting his calories from milk, his appetite will improve. No more than 16 ounces a day - period. I don't care how much he carries on. You need to be his parent here, not his friend. The rest of beverage is water only