In a large skillet over medium-high heat, cook the ground beef and chopped onion until the beef is fully browned. Break it apart with a spoon as it cooks. Add the minced garlic and cook for 1 more minute until fragrant. Drain off any excess grease.

Transfer the cooked beef mixture to the basin of a 6-quart or larger slow cooker. Add the rinsed and drained kidney beans, black beans, and pinto beans. Pour in the diced tomatoes (undrained), tomato sauce, and beef broth.

Sprinkle the chili powder, cumin, smoked paprika, oregano, salt, black pepper, and optional cayenne pepper over the ingredients in the crockpot. Stir everything together until well combined.

Cover the crockpot and cook on LOW for 6-8 hours or on HIGH for 3-4 hours. For the best flavor, the LOW setting is recommended.

Stir the chili one last time before serving. Serve hot with your favorite toppings like shredded cheese, sour cream, and fresh cilantro.