Preheat oven to 350 degrees. In a bowl of an electric mixer or food processor, combine flour, sugar and salt. Add butter and orange zest, mixing until well combined and dough is formed. Dump dough into middle of the tart shell pan. Using your hands press the dough from the middle outward, spreading the dough across the bottom and up the sides of the pan. (You can use a sheet of plastic wrap between dough and hands).