In recent interviews senior Audi people have said they'll be developing more efficient diesel engines and lighter cars to reduce their corporate CO2 emissions below the threshold where the EU will impose punitive taxes on them.
So they don't want volume sales of high CO2 vehicles, like an RS3 would be. The TT RS, on the other hand, is a small niche market and they can take the small hit that would make on their corporate CO2 level.
So IMHO the chance of an RS3 is zero.