Dont bother with roof bars as they do leave residual paint/lacquer marks as used before even with rubber & need polishing out, also dont bother with rear mounted strapped holder as you cant use with spoilers full stop cause the spoiler will get damaged & isnt strong enough to take the pressure & weight.
The problem with the S3 is you cant get a valance for the detachable towbar, so you either have to cut a piece out or have it fitted so its coming from under the bumper & always on or change the valance for the sline that has the proper trim to remove so you can fit a detach towbar for bike rack as I have on mine, its pretty much invisible when not in use as the ball part just comes of & trim covers hole plus the electrics socket swings up into place & doesnt budge or make any noise at all when not in use.
I have a Euroclassic 902 IIRC & these are as already advised brilliant cause 30 seconds & she's on, bikes easily sit on there & have rear lights.
Now with the 8P there is a kit to properly integrate with the cars electronics & parking sensors, if you go after market then they will most likely tap into rear lights & also interfere with sensors, I fitted my kit with parts got from ebay cause the kit is about £700ish as it has 2 looms, 1 control module, towbar. Note bumper has to come of as the rear reinforcement bar is removed & the towbar is fixed in its place,
Its the only safe & best option to use, trust me on this.