It can be all three, or just 2, or just beef. Here's my grandma's recipe.
1 pound ground chuck, or equal parts beef/pork/veal, or 50/50 of beef/pork or veal.
1/2 c. breadcrumbs
1/4 c. (or more to taste) grated parmesan cheese
1 t. salt
1/2 t. black pepper