Heat oil in large nonstick skillet over medium-high heat. Add onion and green pepper; cook until softened, about 10 minutes.
Add chili powder, salt, cumin and garlic powder; cook 1 minute. Stir in tomato sauce. Bring to simmering. Add kidney, pinto and cannellini beans; heat through, 5 minutes.
Meanwhile, warm tortillas according to package directions.
Mash beans. Spoon about 1/3 cup bean mixture into each tortilla. Serve with lettuce, tomato and cheese. Makes 10 soft tacos.