Soak the dried red kidney beans, black beans, and pinto beans overnight in separate bowls.

Drain and rinse the soaked beans.

Heat a little oil in a large pan or Dutch oven over medium heat.

Add the chopped onion and garlic to the pan and cook for about 5 minutes, or until softened.

Stir in the ground cumin and chili powder and cook, stirring constantly, for 1-2 minutes, until fragrant.

Add the drained beans and chicken stock to the pan and bring to a boil.

Reduce the heat and boil vigorously for about 10 minutes.

Transfer the bean mixture to a crockpot.

Cover the crockpot and cook on low heat for 10 hours, or until the beans are tender.

About 20 minutes before serving, add the chopped yellow bell pepper to the crockpot and stir to combine.

Season the chili with salt and pepper to taste.

Stir in the sugar to balance the flavors.

Serve the mixed bean chili immediately.