Heat olive oil and butter in a large saucepan over medium heat. Saute onion, celery and carrots until soft. Add veal, pork, beef and pancetta, and cook until evenly brown, 15 to 20 minutes. Pour in milk, and cook until liquid has evaporated, about 15 minutes. Stir in tomato sauce and beef broth. Cover, and simmer 2 1/2 hours.