"My dearest 80-something friend Betty gave me this sensational, hearty recipe for a 3-bean hot dish. She's Finnish, I believe this has roots to home, and it's really tasty! My hubby, who hates beans, demanded I get the recipe. We first devoured it at Betty and Jim's 60th wedding anniversary! Perfect for potlucks."
Place bacon in a large skillet and cook over medium-high heat, turning occasionally, until evenly browned, about 10 minutes. Drain bacon slices on paper towels. Crumble when cooled. Wipe out skillet with a paper towel.
Cook and stir ground beef in the hot skillet until browned and crumbly, 5 to 7 minutes. Drain and discard grease. Add onion and cook until clear, about 5 minutes more.
Combine cooked bacon, cooked beef, pork and beans, kidney beans, butter beans, ketchup, brown sugar, vinegar, and mustard in a slow cooker. Stir to combine. Cook on High until heated through, about 1 hour.