Heat oven to 350°F. In 2-quart saucepan, mash 2 cups of the strawberries; stir in cornstarch and granulated sugar. Cook over medium heat, stirring constantly, until mixture boils. Boil and stir 1 minute. Carefully stir in blueberries, raspberries and remaining strawberries. Pour berry mixture into ungreased 8-inch square (2-quart) glass baking dish or 9-inch pie plate.