Heat a large skillet over medium heat and add the ground beef. Cook until browned, breaking it up with a spoon as it cooks.
Add the chopped onion and minced garlic to the skillet. Cook for 3 to 4 minutes until the onion softens.
Stir in the chili powder, cumin, cayenne pepper, salt, and black pepper. Cook for 1 minute to blend the spices with the meat.
Transfer the beef mixture to the slow cooker. Add the rinsed black beans, kidney beans, and pinto beans.
Pour in the crushed tomatoes and beef broth. Stir everything together until well combined.
Cover and cook on low for 6 to 8 hours. Stir once halfway through if possible.