The top section of the bumper is exactly the same on an A3 (pre 8P3) , S3 ,RS3 .
The only difference is the lower diffuser section what actually makes a huge difference to the rear .
I just realised that the original RS3 won't fit as you have a three door . You could look for an aftermarket one .If your feeling brave get an S3 lower section and modify it to fit the RS3 one .