Special equipment: 1 (10-inch) square cake pan with 3-inch high sides 1 (8-inch) square pan with 3-inch high sides 2 cardboard cake boards, 1 (10-inch) square and 1 (8-inch) square 3 pairs of disposable latex gloves Small moon and star-shaped cookie cutters
Preheat oven to 350 degrees F.
Butter and flour cake pans. Combine 2 cake mixes, 2 2/3 cups of water, 6 egg whites, and 1/4 cup of oil in large bowl. Beat for 3 minutes, or until well blended. Pour batter into 10-inch pan.
Combine 1 cake mix, 1 1/3 cups of water, 3 egg whites, and 2 tablespoons of oil in same bowl. Beat for 2 minutes, or until well blended. Pour batter into 8-inch pan. Bake 10-inch cake for 1 hour 15 minutes, and 8-inch cake for 55 minutes, or until toothpick inserted into center of cakes comes out clean. Cool cakes in pans on cooling racks for 30 minutes. Invert cakes onto cooking racks and cool completely.
Stir preserves in a heavy small saucepan over medium-high heat until beginning to boil. Strain preserves into small bowl to make a glaze. Spoon 1 tablespoon of glaze into center of each cardboard round. Place boards, glaze side down, atop cakes. Invert cakes onto boards. Brush remaining glaze over cakes.
For The Fondant:
Sprinkle work surface and rolling pin with powdered sugar. Soften fondant in the microwave at 30-second intervals, until workable. Wearing gloves, knead green food coloring, 1 drop at a time, into fondant from 1 box. Roll out green fondant into 18-inch square that is 1/4-inch thick, rotating to prevent sticking. Pierce bubbles that appear in fondant. Slide hands under fondant and drape over 10-inch cake; smooth over surface. Trim excess fondant from around base of cake. Brush off excess powdered sugar.
Knead trimmings together; wrap in plastic. Knead pink food coloring, 1 drop at a time, into 1/2 cup of white fondant. Knead blue food coloring, 1 drop at a time, into 1/2 cup of white fondant Wrap pink and blue fondants separately in plastic; set aside. Knead yellow food coloring, 1 drop at a time, into remaining fondant from second box. Roll out yellow fondant into 15-inch square that is 1/4-inch thick. Slide hands under fondant and drape over 8-inch cake; smooth over surface. Knead trimmings together; wrap in plastic.
Place 8-inch cake atop 10-inch cake. Roll out colored fondant trimmings separately into 1/4-inch thickness. Cut out shapes from fondant with cookie cutters. Moisten shapes with water, then dab gum arabic onto moistened side; press shapes onto cakes to adhere.