Heat olive oil in a large skillet over medium heat.
Add the chopped onion and sauté for 2–3 minutes until softened.
Add minced garlic and cook for another 30 seconds until fragrant.
Add ground beef to the skillet and cook until browned, breaking it apart with a spoon.
Drain excess fat if necessary.
Transfer the cooked beef mixture to the slow cooker.
Add crushed tomatoes, tomato paste, and beef broth to the slow cooker.
Stir in kidney beans, black beans, and pinto beans.
Add bell pepper, chili powder, cumin, smoked paprika, oregano, salt, black pepper, and red chili flakes.
Stir well to combine all ingredients evenly.
Cover and cook on low for 6–8 hours or high for 3–4 hours.
Stir occasionally if possible.
Taste and adjust seasoning before serving.