You would quilt 3 separate quilts to within 1/2 inch from where they are to be joined. After all the sections are quilted, you join them together and use a sashing strip to hide the seam on the back. Search Quilt as You Go for a block by block style or even Quilting in Sections and you should find examples of what it looks like. You only have the entire quilt on the machine when you are putting the sections together and of course putting the binding on.
Sharon Pederson & pair of ladies I forgot the names of did episodes on Simply Quilts years ago and I'm sure there are others. Check YouTube for videos.
I have quilted block by block which is very similar to the quilting in sections & it was very easy. I quilted each block and then used sashing front & back to put them together row by row. It was a lot of hand sewing on the back to finish but if you use a matching thread color, you could probably machine finish it easily enough. I was a new quilter at the time so I followed the directions I had seen.