You can make the cake in two batches if you only have three cake pans. Mix one cake mix with ½ cup flour, 5 egg whites, ¾ cup milk, ½ cup sour cream, and ½ cup vegetable oil. Divide the batter into three bowls and use one color for each. Bake the cakes as instructed. Once the cakes are cool, remove them from the pans and use the same pans to make the other three layers using the rest of the colors.