Mix flour, sugar, and salt in a bowl. Using a pastry blender or 2 knives, cut in margarine until mixture is crumbly.
Spoon 1 tablespoon water into mixture at a time and blend with a fork after each addition; pastry should be just moist.
Roll our dough on a lightly floured surface into a sphere 2 inches larger than pie plate. Wrap dough around rolling pin and unroll into an 8 or 9-inch pie plate. Trim edges, fold under, and flute using fingers.