1Line a 9-inch square pan with foil, with ends of foil extending over sides. Arrange half the graham pieces, in 3 rows of 7 pieces each, on bottom of pan.
2Pulse fruits in blender or food processor just until blended--some chunks should remain.
3Spoon fruit into medium bowl; stir in yogurt and whipped topping. Spread over graham pieces in pan; top with remaining graham pieces, aligning over graham pieces on bottom layer, so they form squares.
4Freeze for 4 hours. Use foil "handles" to lift dessert from pan; cut between grahams into 21 bars. Let stand 5 minutes before serving.