Choose your cooking method: either a slow cooker or an oven. Opt for a large, deep casserole dish or a heavy cast iron Dutch oven if baking. Slow cookers work well for serving directly and maintaining warmth for extended periods.
Slice the raw bacon into small bits and render it over medium heat in a cast iron skillet or Dutch oven. Stir frequently with a wooden spoon. For efficiency, cut the bacon directly into the pot using kitchen scissors.
Once the bacon crisps, extract it using a slotted spoon, placing it in a bowl. Retain the rendered fat in the pan. Add the diced onion and sauté for about 5 to 6 minutes until it softens and becomes translucent. Incorporate the minced garlic, cooking for an additional minute.
Add the ground beef to the pan and cook thoroughly until browned, approximately 6 to 8 minutes. Return the cooked bacon to the pan with the beef, onion, and garlic.
Pour in the undrained baked beans, then stir in the drained kidney beans, cannellini beans, and black beans. Mix in the brown sugar, apple cider vinegar, ketchup, and yellow mustard, blending everything together evenly.
Transfer the mixture into your chosen cooking vessel. A standard 9x13-inch dish is generally insufficient in depth, so use a larger container.
For slow cooker preparation, set on low and cook for 2 hours. For oven baking, cover and bake at 350°F for 1½ hours. Serve while hot.