Preheat oven to 350 degrees F (175 degrees C). Line 12 cupcake cups with paper liners. Grease top of cupcake pan to help remove baked cupcakes.
Place the cake mix, water, vegetable oil, and eggs into a large mixing bowl, and mix with an electric mixer on slow speed for 30 seconds to combine ingredients. Increase speed to medium, and beat 1 more minute, scraping down the sides of the bowl occasionally. Spoon the batter into the lined cupcake cups, filling them to the brim. Cupcakes will rise and bake over top of the liners, making wide, round tops.
Bake in the preheated oven until a toothpick inserted into the center of a cupcake comes out clean, about 25 minutes; check after 20 minutes. Let the cupcakes cool in the pan completely. Gently run a knife underneath the cupcake tops to separate them from the pan so cupcakes can be removed, but leave in the pan.
Cut the green stem end from each berry, leaving a flat platform on the end. Slice each berry into quarters by cutting in half, then rotating berry and cutting in half again crosswise. There will be a total of 60 triangle-shaped berry pieces.
To decorate, cut the wide rim of a cupcake top into a star shape with a sharp paring knife by cutting out 5 shallow triangles, evenly spaced around the top. Leave the middle of the cupcake uncut, and discard cut-out portions. Frost the cupcake with the white frosting, following the star shape of the top and leaving the cut-out portions unfrosted. Place 3 blueberries into the center of the cupcake, and place 5 strawberry slices surrounding the blueberries, cut sides down and points facing outward, to make a large strawberry star. Repeat for remaining cupcakes.