Crisp The Bacon. In a large skillet, cook the bacon over medium heat until crispy and golden brown. Add the cooked bacon to the crock of your slow cooker and reserve at least 2-3 Tablespoons of the bacon fat in the skillet. (Feel free to keep all the bacon fat for the richest flavor, or you can discard it/reserve the remaining bacon fat for another use.)

Soften The Onion. Add onion to the skillet with the bacon fat and cook over medium heat 4-5 minutes, or until the onion is softening and partly translucent. Stir regularly to scrape up as many browned bits from the pan as you can.

Mix Up The Beans. Add the softened onion & the bacon fat to the slow cooker along with the butter beans, kidney beans, and Great Northern beans. Add brown sugar, ketchup, barbecue sauce, molasses, mustard, Worcestershire sauce, and black pepper. Pour in the water and stir until the mixture is uniform and well blended. (This helps you avoid scorching.) Cover with slow cooker lid. 

Cook the beans 3-4 hours on HIGH or 7-8 hours on LOW.

Taste & Adjust. Before serving, taste the beans and adjust the seasoning as desired, adding more brown sugar, mustard, barbecue sauce, Worcestershire sauce, pepper, or salt, as needed.

Serve & Store. Enjoy the beans warm. Chill leftovers and store in the fridge in an airtight container 3-4 days.