Combine first 3 ingredients; cut in shortening with a pastry blender until crumbly. Sprinkle 1/4 cup cold water, 1 tablespoon at a time, over surface; stir with a fork until dry ingredients are moistened. Shape into a ball; chill.
Roll pastry to 1/8-inch thickness on a lightly floured surface. Fit into a 9-inch pieplate; trim off excess pastry along edges. Fold edges under, and crimp.