In a medium saucepan, combine the 2 cups raspberries, the 1 cup blackberries, the 1 cup blueberries, and the sugar. Toss to coat berries. Cover and cook over medium heat for 5 minutes. (Allow about 10 minutes if using all frozen fruit.) Uncover; stir gently. (Berries should be soft, but still hold their shapes. Berry juice and sugar will have formed a syrup.) Remove the saucepan from heat.