Preheat the oven to 350 degrees F (175 degrees C).
Heat a Dutch oven over medium heat. Use a large pot or saucepan if you do not have a Dutch oven. Add the beef, bacon and onion. Cook and stir until beef is evenly browned and crumbled. Drain off grease.
Stir in the baked beans, kidney beans and Great Northern beans. Combine the ketchup, brown sugar, salt, mustard powder, vinegar and molasses; stir into the beans. If you are not using a Dutch oven, transfer to a casserole dish.