Straight cut, recessed into the bumper is the nicest on an a3 imo.
Prawns old a3, now pierres, is the perfect example
Have you ever seen it in real life?
Personally; I'm not a huge fan. It is impossible to keep an exhaust within the small confines of that cutout, and as such it burns around the bumper and melts the lower trim. I'm sure there would be a way of fitting it to help, but every time I've seen it the exhaust hasn't been straight in the cutout, or has been a bit messy.
I much prefer a twin slash cut exhaust. And they're easier to get right.
My last A3 had a 2.5" single rolled exit, looked good to be fair. Shame the cutout was for when it had a twin pipe miltek.
Before that it had a straight cut twin exit...Which sat wonkey and looked like ****.
This is what I have on my car at the moment (modeled on Prawns car)
And a slightly different fitment on my car -
You can see it has been cut by Prawn to match the contour of the rear bumper.
It's all personal choice really, but that's pretty much all of your options in one post