Combine first 6 ingredients in a medium bowl; make a well in center of mixture. Combine buttermilk, oil, and egg; add to dryingredients, stirring just until moistened. Gently stir in squash, corn, and red pepper. Spoon batter into muffin pans coated with cooking spray, filling three-fourths full. Bake at 400° for 15 minutes or until golden. Remove from pans immediately.