Dealer part tbh... seen too many pattern parts fail... I used a pattern part inner boot once and it lasted 6 months..
If the A3 quattro in question has a 5 speed box based on an 02M box or a 6 speed box then yeah, boots 'should' be the same as they use the same diameter joints however as I say, use a dealer part and you will get the right one anyway...
Only exception is GKN boots... they seem to last well...