Preheat your oven to 350°F (175°C). Grease and flour three 8-inch round cake pans.
In a large mixing bowl, combine the cake mix, water, vegetable oil, and eggs. Beat with an electric mixer on medium speed for about 2 minutes until well blended.
Divide the batter evenly into three bowls. Leave one bowl as is for the white layer. In the second bowl, gently fold in the blueberries until evenly distributed. In the third bowl, gently fold in the sliced strawberries.
Pour the white batter into the first prepared cake pan. Pour the blueberry batter into the second pan and the strawberry batter into the third pan.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted in the center comes out clean. Let the cakes cool in the pans for 10 minutes before transferring them to wire racks to cool completely.
While the cakes are cooling, prepare the whipped cream frosting. In a mixing bowl, combine the whipped cream, powdered sugar, and vanilla extract. Beat until soft peaks form.
Once the cakes are completely cooled, place the white cake layer on a serving plate. Spread a layer of whipped cream frosting on top. Add the blueberry layer and spread more frosting on top. Finally, add the strawberry layer.
Frost the top and sides of the entire cake with the remaining whipped cream. Decorate with red, white, and blue sprinkles if desired.
Chill the cake in the refrigerator for at least 30 minutes before serving to allow the layers to set.