Since you are sending the quilt out to be longarmed, are you aware that the backing seams should be across the width of the quilt? Traditionally the seams run from top to bottom. However, when a quilt is rolled on a frame, vertical seaming results in a buildup of bulk where the seams are. That is why longarmers typically ask quilter to run their backing seams from side to side. This keeps the quilt rolling evenly on the rollers.
I believe it is also better, when quilting on a frame, to iron the backing seams open. Not sure if that matters much or not, though. I think it is also best to allow .5" for seams in the backing fabric, to keep them strong.
Traditionally, in a situation such as yours, one would use a full width of fabric in the middle and add equal amounts to each side to get the total width. However, 3 panels of equal width would work fine too.