Actually I always fold all 3 layers and put a safety pin at the center of all 4 sides of each piece. Then when I sandwich them I line up the safety pins, but this does not seem to be fail proof to keep a pieced back from shifting.
I've done the piecing, where I lay 2 pieces rt. sides together and then sew down both side seam, and the cut up the center of 1 side. Also added a wide strip between two 42-44 widths and sometimes put wide bands at the top and bottom. But the biggest problem is still shifting.
I decided to go out and buy a 108 " back to cut this quilt's back.