For the filling, boil the sweet potatoes until tender. Peel and mash. Heat your oven to 350ºF. Combine the margarine, granulated sugar, brown sugar, salt and nutmeg in a large bowl. Beat at medium speed, with an electric mixer until smooth. Beat in the eggs and egg whites. Beat in the milk and vanilla slowly. Spoon into 3 unbaked pie shells, using about 4 cups of filling per shell. Bake at 350ºF. for 50 to 60 minutes or until set. cool to room temperature before serving. Refrigerate the leftover pie.