Finely grate zest from 2 oranges and 1 grapefruit into a small bowl and stir in sugar.
Cut peel, including all white pith, from oranges and grapefruit with a sharp knife, then, working over a large bowl, cut segments free from membranes, letting segments fall into bowl.
Squeeze enough juice from membranes to measure 1/2 cup, then add to citrus segments with grapes and 3 tablespoons citrus sugar.
Toss fruit gently and serve with remaining sugar on the side.
Note: • Compote can be made 1 day ahead and chilled, covered.
Keep remaining citrus sugar in a sealed plastic bag at room temperature.
