1. Preheat oven to 375 degrees F. Spray a 2 1/2 quart baking dish with nonstick spray.
2. Heat the oil in a nonstick skillet over medium-high heat. Add the onion and bell pepper; cook, stirring frequenly , until tender, about 6 minutes. Transfer to a large bowl. Add teh black, red and pinto beans, the BBQ sauce, mustard, sugar and chili powder; mix well.
3. Spoon the bean mixture into the baking dish. Bake, covered, 45 minutes. Uncover and bake until the beans are thickened slightly and bubbly, about 15 minutes longer.