Preheat oven to 325°F. In a large skillet, cook beef and onion over medium heat until beef is no longer pink, 5-7 minutes, breaking up beef into crumbles. Drain excess fat.
Add pork and beans, kidney beans, butter beans, brown sugar, ketchup, bacon, cider vinegar, mustard and salt to the skillet. Stir to combine thoroughly.
Transfer mixture to a greased 2-quart cast-iron pan or baking dish.
Bake uncovered for 45-60 minutes, until beans reach desired thickness and are bubbling hot.