You don't have to cut the top into thirds in order to quilt it in sections. What you do instead is cut the *batting* into thirds. This reduces the bulk considerably, especially when quilting the middle. There are some tricks to doing this so you don't end up with a mess. Try searching for "quilt in sections" on the QB and you should pull up some old threads on this technique.
Edit: I found this old thread with some information.
http://www.quiltingboard.com/main-f1...s-t268061.html