Preheat the oven to 350°F. In a large skillet, cook the bacon over medium heat until lightly crisp. Transfer to a paper towel-lined plate to drain. In a separate skillet, cook the ground beef over medium heat, breaking it into small pieces as it browns. Drain any excess grease. In a 5-quart Dutch oven, combine the cooked bacon, ground beef, onion, baked beans, kidney beans, and butter beans. Stir in the brown sugar, ketchup, and dry mustard until well combined. Cover and bake for 1 hour. Reduce the oven temperature to 200°F and continue baking for an additional 20 minutes. Serve warm.