You can get one for the RS3, however it is the same as that for the S3 and there is a slight problem in so far as while it fits the rear, it wont fit the front as the brake calipers are too big. If you puncture on the front you would have to change rear onto front, then fit space saver to rear. Probably no bad thing as at least you would have a decent front tyre, albeit marginally narrower on the front for the steering
A lot of faffing around on the RS3, if you got a puncture in the front you'd have to swap a good back tyre onto the front and put the space saver on the back, as i doubt the space saver would clear the front Brembo Caliper.
My space saver is 125/70/19, there is slightly less than 2'' clearance of the calliper with the 20" wheels fitted,if its OK with John we can try the space saver next time i'm at his so at least then we will know for sure.