For filling, in a 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.)
Preheat oven to 375 degrees F. Prepare and roll out one portion of the Pastry for Lattice-Top Pie. Line a 9-inch pie plate with the pastry.
Stir berry mixture. Transfer berry mixture to the pastry-lined pie plate. Trim bottom pastry to 1/2 inch beyond edge of pie plate. Roll out remaining pastry; cut into 1/2-inch-wide strips. Lay half of the pastry strips over the filling at 1-inch intervals. Lay remaining pastry strips at 1-inch intervals perpendicular to the first strips. Fold the bottom pastry up over the pastry strip ends to seal; crimp edge.
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 bowl, combine flour and salt. Using a pastry blender, cut in shortening until pieces are pea size. Sprinkle cold water, 1 tablespoon at a time, over flour mixture, tossing with a fork and pushing moistened pastry to side of bowl after each addition. Divide pastry in half; form halves into balls.