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.

Trim crust from bread. Arrange 4 slices close together to form a square. Cover with 4 more slices. Using an 8-inch diameter dish such as a 2-quart souffle dish or 1-1/2-quart casserole, trim the bread to fit into the dish. Discard bread trimmings. Cut the remaining 3 bread slices inhalf crosswise (not diagonally).

Line the souffle dish or casserole completely with plastic wrap, leaving a generous overhang and smoothing out the wrap as much as possible. Place 4 of the trimmed bread slices in the bottom of the lined dish. Place the 6 half-slices around the edge of the dish.

Using a slotted spoon, spoon the warm berries onto the bread slices. Top with the remaining 4 trimmed bread slices. Drizzle with the berry syrup. Fold extended ends of plastic wrap over mixture, adding more wrap if necessary to cover completely.

Place a small plate on top of wrapped dessert; weight with two 16-ounce cans. If the dish s shallow, set it on a large plate to catch any overflow juices. Refrigerate the dessert overnight.

To unmold, remove the weights and small plate. Peel back the plastic wrap. Invert the dish onto a serving platter; remove dish and plastic wrap. Cut dessert into wedges. Garnish with fresh berries and mint sprigs and pass whipped cream, if you like. Makes 6 to 8 servings.