I went through swapping my non sline 5 door bumpers to sline and confirm the rear bumpers are different on the 3 door and 5 door. To answer your question. As long as the bumpers are from the same 3 or 5 door it will fit.
If i understand right from what you've just said my friend, the back bumper of an S3 Sportback 5 door can be fitted to a simple A3 Sportback 5 door right ?