Place bacon in a large, deep skillet. Cook over medium-high heat until evenly brown. Drain, break into medium pieces, and set aside. In the same skillet, brown beef and onion. Drain fat.

Preheat the oven to 350 degrees F (175 degrees C).

Place bacon and beef mixture in a large bowl and stir in baked beans, kidney beans (with liquid), butter beans (with liquid), brown sugar, ketchup, vinegar, and mustard. Mix well.

Spoon casserole mixture into a 9x13-inch baking dish and bake uncovered in the preheated oven for 1 hour.