Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the cooked kidney, pinto, and black beans.
Add the corn, green bell pepper, onion, and diced tomatoes to the bowl.
In a separate bowl, mix together the barbecue sauce and brown sugar.
Pour the barbecue sauce mixture over the bean and vegetable mixture.
Season with salt and pepper to taste.
Stir all ingredients until well combined.
Transfer the mixture to a baking dish.
Bake in the preheated oven for 30-40 minutes, until heated through.
Remove from the oven and let cool slightly before serving.