In a medium saucepan combine sugar, cornstarch, and salt. Stir in water and 1 cup of the blueberries. Cook and stir over medium heat until thickened. Remove from heat. Stir in remaining 2 cups blueberries and butter until butter is melted. Cool completely. Pour into pie shell. If desired, cover and chill in refrigerator until ready to serve. Serve with whipped cream. Makes 8 servings.