Remove silver membrane from the back of ribs. Cut ribs into sections that have 3-4 ribs per section and massage rub mixture into the ribs on all sides.
Add ribs, onion, sliced garlic, and water to slow cooker.
Cook ribs for 4 hours on high or 8 hours on low, or until ribs are fork-tender.
Preheat broiler (or grill). Combine ketchup, chili sauce, and barbecue sauce.
Brush ribs with sauce mixture and broil (or grill) 3-5 minutes or until charred.