A 3-layer cake, especially one that you want to showcase for a special occasion, needs to look like a masterpiece, not a mess. It's somewhat of an art to stack a 3-layer cake successfully. You have to assess each layer and decide which would fit best where. If a layer has a crown on top, you may have to alter it to allow it to stack well with the other layers.
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If you have a cake layer with a pronounced dome on top, you will need to shave the dome off so that the layer is even. Place the cake layer flat on a cutting board, and cut the dome off evenly with a serrated knife.
Dab 1/2 tsp. of frosting onto the cake platter before placing the thickest layer of cake on it, top side up. The dab of frosting will help anchor the cake to the serving dish.
Frost the bottom layer of the cake with a plastic spatula. Place extra frosting in any spots that need to be evened out.
Position the second layer, top side up, on top of the bottom layer. Frost the entire second layer. Smooth the seam of frosting between the first and second layers with the spatula. Build up any low or lopsided spots with frosting.
Place the third layer, top side up, on top of the cake. Place a large dollop of frosting on top of the top layer, and smooth it gently over the top and down the sides. Don't allow the spatula to dig into the cake, or crumbs may get into the frosting.