In a saucepan, combine the sugar, cornstarch and water until smooth. Bring to a boil; cook and stir for 2 minutes or until thickened. Reduce heat; stir in raspberries. Simmer, uncovered, for 5 minutes or until heated through. Remove from heat. Cool to room temperature. Add oranges, pears and grapefruit; stir gently to coat. Chill leftovers.