Prepare pastry. On a lightly floured surface, roll one ball of pastry into a 12-inch circle. Line a 9-inch pie plate with the pastry circle. Set aside.
In a large mixing bowl, combine sugar, cornstarch, and lemon juice. Stir in blueberries, raspberries, and blackberries. Toss gently. Spoon into pastry-lined pie plate. Trim pastry even with edge.
On a lightly floured surface, roll out remaining pastry into a 12-inch circle. Cut slits to let steam escape. Place pastry on filling. Trim top crust to 1/2 inch beyond edge of plate. Fold top pastry under bottom pastry and crimp edge. Brush top with milk and sprinkle with the coarse sugar. Cover edge of crust with foil to prevent overbrowning.
Bake in a 375 degree F oven for 25 minutes. Remove foil. Bake for 25 to 30 minutes more or until top is golden. Cool on a wire rack. Makes 8 servings.