Preheat oven to 425 degrees (to lower to 375 degrees after pie bakes for 15 minutes.
Remove crusts from refrigerator and let them come to room temperature.
Line pie pan with crust as directed for a 2 crust pie on the Pillsbury box.
In a large bowl or pot, add rinsed and well drained blueberries, raspberries, sliced strawberries, and rhubarb. (6-7 cups total.).
In a medium bowl whisk together the sugar, cornstarch, flour, and cinnamon.
Pour sugar mixture onto berry mixture. Add lemon juice. Fold together gently but thoroughly and set aside to blend for 15 minutes. Stir again and pour into deep dish glass pie pan.
Dot top of filling with small butter cubes. Top with second crust that has been brushed with egg and water mixture and sprinkled with coarse sugar.Make sure to cut air holes to allow steam to escape.
Place on a pizza or cookie pan to catch drips. Place in oven on middle rack. Bake at 425 degrees for 15 minutes; then lower temperature to 375 degrees for 45-50 minutes more. Top with aluminum foil if crust browns too fast.
Remove from oven and let cool for at least an hour before serving. Serve with ice-cream or whipped cream or a glass of ice-cold milk.