Meanwhile, place raspberries in food processor or blender; cover and process until smooth. If desired, place strainer over 1-quart saucepan; pour raspberries into strainer. Press berries with back of spoon through strainer to remove seeds; discard seeds. Stir 3 tablespoons sugar and cornstarch into raspberries in saucepan. Cook over medium heat, stirring constantly, until mixture boils. Place in freezer to cool quickly, about 5 to 10 minutes.