Preheat oven to 350° F. Spray muffin pans with non-stick cooking spray. Prepare cake mix according to package directions.
Fill 7 of each size muffin molds half full. Bake small cupcakes 10 minutes, medium cupcakes 15 minutes and large cupcakes 22 minutes or until toothpick inserted in center comes out clean. Let cool in tins 10 minutes. Remove and cool completely on wire rack.
To assemble mini cakes, make sure all cupcakes are level. Place largest cupcake on plate, top side up, and frost top only. Place medium cupcake, top side up, in center of the large cupcake. Frost top only. Repeat with the smallest cupcake. Continue to build the 3-tiered mini cakes until all 7 are prepared.