The hardest part about making a three-tier cake is the weight. The weight from the upper layers can cause the lower layers to collapse. The easiest way to avoid this issue is to place dowel rods through the inside of the cake, which will support the weight of the upper tiers and prevent the cake from falling apart. Use another dowel rod to keep the entire cake from slipping during transit.
Cut eight dowel rods with the serrated knife to a length of 3 inches. Cut one dowel rod to a length of 8 inches.
Prepare the cake mixes according to the package directions. Pour the batter evenly between the three cake pans. Bake according to package directions. You may have to increase baking time due to the thickness of the cakes. Allow to cool completely.
Place the 12-inch cake onto the centre of the cake drum. Ice the cake with butter cream frosting. Place the other two cakes onto cardboard circles. Frost them, and chill them in the refrigerator for two hours.
Cut two cardboard circles equalling 9 and 6 inches in diameter. Place the 9-inch circle in the middle of the 12-inch cake. Mark the placement of the cake around the top of the 12-inch cake with a knife. Remove the cardboard circle.
Stick four short dowel rods into the 9-inch circle inside the centre of the 12-inch cake. Place the 9-inch cake in the middle of the 12 inch cake, using the outline as a guide for placement.
Place the 6-inch circle on the top of the 9-inch cake and mark the placement of the cardboard with a knife. Remove the cardboard. Place four short dowel rods in the 6-inch circle inside the 9-inch cake. Press the rods down until they are at the same level as the cake itself.
Place the 6-inch cake on top of the 9-inch cake inside the marked area. Place the long dowel rod through the entire cake, tapping lightly with a hammer to anchor it in place. Push the rod all the way to the bottom of the cake with your fingers. Cover the hole in the cake with frosting. Decorate the cake with fondant or other icing and embellishments to finish.