In a microwave-safe 1-quart casserole combine the undrained chili, barbecue sauce, and mustard. Place frankfurters on top of chili mixture. Cover and microwave on 100 percent power (high) for 4 to 5 minutes or until mixture is heated through, stirring once. Place frankfurters on buns. Spoon chili mixture atop. Makes 4 main-dish servings.