Preheat the oven to 350 degrees Fahrenheit and grease the 9x13 pan with non stick cooking spray.
Add 3 eggs, 1 cup water, .5 cup oil and boxed cake mix to mixing bowl.
Whisk ingredients together until well combined.
Pour cake batter into greased pan and bake for about 35 minutes at 350 degrees Fahrenheit.
Remove from oven and allow to cool.
While cooling, wash and dry blueberries and strawberries. Slice strawberries into thin slices.
Once cake is cool, spread 8 oz of whipped topping evenly on top of cake.
Arrange blueberries in a square at the top left of the cake.
Then arrange sliced strawberries in strips across the rest of the cake to resemble the American Flag.
Notes
Tips
Thaw whipped topping - make sure the whipped topping is completely thawed otherwise it wont spread well. Sit it at room temperature for about an hour or in the fridge overnight.
Use fresh berries - while you CAN use frozen berries, they won't be as bright and pretty as fresh berries are.
Evenly sized berries - try to use similarly sized blueberries and similarly sized slices of strawberries to give the cake a straight and clean look.
Bake the cake correctly - make sure that cake is golden brown on the edges and that you can insert a fork or toothpick in the center and pull it out clean.
Storage
Keep in fridge for 3 - 5 days in an airtight container.
Keep in freezer for up to 3 months in an airtight container. Defrost in fridge overnight.