I finally have a few minutes, so I will describe the method I like for quilting in thirds.
Layer as usual. Baste only the middle third of the quilt.
Fold backing fabric and top back about halfway so batting alone is exposed. Make a wavy large "S" shaped cut through the batting about one-third of the way from the edge. The S should be about 6 inches from side to side. (Cutting wavy like this means later on the quilt will not develop a fold line as might be the case if you cut in a straight line.)
Before removing the righthand third of batting, make registration marks on the batting with a permanent marker so later on you can re-assemble the batting *exactly* as it was originally. Also mark the top righthand side to help you later with re-positioning.
Remove the right third of batting, lay the backing fabric and top back out and pin them together.
Do the same on the lefthand side of the quilt.
You end up with much less bulk under the arm of your machine while you are quilting the middle. Leave about 4 inches free at each side of the middle third to allow space for re-assembling the batting.
Once the middle third is quilted, remove pins from right side, fold backing fabric and top out of the way again, and re-assemble the batting on that side. Best way is to hand stitch the batting pieces together using a tailor tack stitch; however, it's also possible to just machine zigzag the pieces together using a long and wide zigzag stitch.
Baste the righthand third of the sandwich together and quilt. Then do the same with the lefthand third.