In a small bowl, stir the granulated sugar and cornstarch together.
Put blueberries in a medium bowl.
In a large bowl, combine the strawberries and cherries together.
Stir 1/4 cup of cornstarch mixture into blueberries.
Stir the rest of the cornstarch mixture into the strawberries and cherries.
Add 1 teaspoon of lemon juice to each fruit mixture
Place in the refrigerator while taking out one roll of dough.
Preheat oven to 400°F
Place your pie dough into a 9x2 in pie dish. Tuck it in with your fingers and smooth the bottom down. To create the divider, fold aluminum foil several times into a 2-inch wide strip. Place into crust, dividing it into 1/3 and 2/3 sections.
Spoon the red fruit into the larger section, leaving any excess liquid in the bowl. Spoon the blueberry mixture into the smaller section and discard any excess liquid. Place the pie in the refrigerator while you work on the flag design.
Remove the other roll of chilled pie dough from the refrigerator. Using a knife or pizza cutter, cut 4 strips that are 2 inches wide and 6 inches long. Using your star cookie cutter, cut as many stars that can fit on the pie. This will depend on the size cookie cutter you use.
Remove the pie from the refrigerator, remove aluminum foil strip, and add top decor to pie. Put the stars over the blueberries and the stripes over the strawberries to create an American flag design. Crimp the pie edges with a fork.
Next, lightly brush the pie crust with the egg wash mixture. Sprinkle with sugar, if desired.
To prevent any pie drippings from burning in your oven, place the pie onto a large baking sheet. Bake for 20 minutes. Keeping the pie in the oven, turn the temperature down to 350°F and bake for an additional 30-35 minutes.
After 55 minutes, remove the pie from the oven and allow to cool for 3 full hours at room temperature before serving. This will allow the filling to thicken up. Enjoy!