For filling, in a large bowl stir together the sugar and cornstarch. Add raspberries, blueberries, and blackberries. Gently toss berries until coated. (If using frozen fruit, let mixture stand for 45 minutes or until fruit is partially thawed but still icy.)
Stir berry mixture. Transfer berry mixture to the pastry-lined pie plate. Trim bottom pastry to 1/2 inch beyond edge of pie plate. Continue as directed for Pastry for Lattice-Top Pie.
If desired, brush top pastry with milk and sprinkle with additional sugar. To prevent overbrowning, cover edge of pie with foil. Place a foil-lined baking sheet on the rack below pie in oven. Bake for 30 minutes (or 50 minutes for frozen fruit). Remove foil. Bake pie for 35 to 40 minutes more or until filling is bubbly and pastry is golden. Cool on a wire rack.
Pastry for Lattice-Top Pie
In a large bowl stir together flour and salt. Using a pastry blender, cut in shortening and butter until pieces are pea-size.
Sprinkle 1 tablespoon of the water over part of the flour mixture; toss with a fork. Push moistened pastry to side of bowl. Repeat moistening flour mixture, using 1 tablespoon of the water at a time, until flour mixture is moistened. Gather flour mixture into a ball, kneading gently until it holds together. Divide pastry in half; form halves into balls.
On lightly floured surface, use your hands to slightly flatten one pastry ball. Roll it from center to edge into a 12-inch circle.
Wrap pastry circle around rolling pin. Unroll pastry into a 9-inch pie plate. Ease pastry into pie plate without stretching it. Transfer desired filling to pastry-lined pie plate. Trim pastry to 1/2-inch beyond edge of pie plate.
Roll remaining ball into a 12-inch circle and cut into 1/2-inch-wide strips. Transfer desired filling to pastry-lined pie plate. Weave strips over filling in a lattice pattern, twisting strips, if desired. Press strip ends into bottom pastry rim. Fold bottom pastry over strip ends; seal and crimp edge. Bake as directed in recipes.