Grease two 10 x 2-inch-round-cake pans; 7 x 2-1/2-inch springform pan. Line bottoms with waxed paper; grease paper. Fill springform with batter 1/2 inch from top. Divide remaining batter equally between 10-inch pans.
Bake the 3 cakes in 350°F oven for 25 to 30 minutes or until 10-inch cakes test clean with wooden pick. Remove 10-inch cakes from oven. Leave 7-inch cake in oven; cover loosely with foil. Bake 25 to 30 minutes longer or until wooden pick tests clean. Cool cakes in pans on wire rack 5 minutes. Remove cakes to rack to cool. Trim 7-inch cake so top is flat.
Place one 10-inch cake, flat side down, on plate. Spread top with 1 cup buttercream. Place second 10-inch cake, flat side up, on top. Spread top with 1 cup buttercream. Trim top of 7-inch layer, to make even. Place, flat side up, on top. Frost cake with remaining buttercream. Decorate with flags and candies.
Beat together confectioner's sugar, butter and vanilla in large bowl until smooth. Keep covered with plastic wrap until ready to use. Makes about 3 cups.