Flatten chicken to 1/4 inch thickness. Place three slices of pepperoni and 1 slice cheese on each. Roll up tightly; secure with toothpick. Place in a greased 11x7 baking dish. Spoon the pizza sauce over the roll-ups. Then cover and bake at 350 degrees for 35-40 minutes. Uncover and top with remaining cheese Bake 5-10 minutes longer or until cheese is melted.