Preheat oven to 375 degrees.  Press 1 pie crust into a lightly greased pie pan. In large bowl, combine blueberries, strawberries, and raspberries.  In small bowl, combine cornstarch and 1/4 cup water and stir until smooth and then add in 3/4 cup of the sugar. Pour cornstarch, sugar and water mixture over the berries and toss. Let stand 20 minutes, stirring occasionally. Pour berries into pie shell and place remaining pie shell over the top and crimp edges together (you can also cut pastry into strips and weave into a lattice pattern). Brush top of pie with beaten egg and sprinkle with 1 tablespoon of sugar.  Cut slits or pattern in the top of pie to vent.  Bake  at 375 degrees for 40-50 minutes until crust is lightly browned. If edges brown too quickly tent pie with foil. Pie is delicious served warm or cold with vanilla ice cream.