Heat a large skillet over medium-high heat until a drop of water sizzles on contact. Add the ground beef, break it apart with a wooden spoon, and cook for 6 to 8 minutes until no pink remains and the meat has some browned bits. Spoon off most of the excess grease, leaving about a tablespoon behind.
Add the diced onion and green bell pepper to the skillet and cook for 4 to 5 minutes until the onion turns translucent and the pepper softens. Stir in the minced garlic and cook for 1 more minute until fragrant.
Add the chili powder, cumin, smoked paprika, dried oregano, cayenne (if using), salt, and black pepper. Stir constantly for about 30 seconds so the spices toast in the warm fat and release their aroma.
Scrape the entire beef and spice mixture into the slow cooker. Add the drained and rinsed kidney beans, pinto beans, and black beans, then pour in the diced tomatoes with their juice, the tomato sauce, and the tomato paste. Add the beef broth, Worcestershire sauce, and brown sugar (if using).
Stir everything together until the tomato paste dissolves and the beans are evenly distributed. Cover with the lid and cook on low for 6 hours, or on high for 3 to 4 hours. Resist lifting the lid during cooking, since each peek adds roughly 20 minutes to the cook time.
After the cook time, stir the chili and taste it. If it is too thin, leave the lid off and cook on high for another 20 to 30 minutes to reduce. If it is too thick, stir in a splash of beef broth or water. Adjust salt and heat to taste before serving.
Ladle the chili into bowls while hot. Set out toppings like shredded cheese, sour cream, diced onion, and tortilla chips, and let everyone build their own bowl.