Heat oven to 350°F. Place mini paper baking cup in each of 24 mini muffin cups. In a medium bowl, beat cake mix, water, 3 tablespoons oil and the egg with electric mixer on low speed 30 seconds, then on medium speed 2 minutes. Divide batter among muffin cups. Bake 11 to 13 minutes or until toothpick inserted in center comes out clean. Cool 10 minutes; remove from pans to cooling racks. Cool completely.