If I understand right, you cut and laid your backing vertically (lengthwise). Therefore, like you said, your backing could only be 88". If you turn the backing horizontally, you might be able to save the backing you cut. You will have to cut off the excess and make that the 3rd horizontal panel, pieced, but it should workout.
This should help
http://www.allpeoplequilt.com/techni...lt-back_1.html
Good luck.