NO. S3 foglights are smaller than the A3 ones and yes its just a scam by Audi to make them slightly smaller so they can charge more when they break. I know for a fact that they dont fit because i had mine pinched, bought some A3 ones they didnt fit so i fitted some PIAA driving lamps instead!! And yes they ROCK!!
PIAA ones fit into the original hole perfectly and look very standard apart from the diachroic(sp) lense which is slightly yellow, but they work even better in foggy conditions without dazzling oncomers.
PIAA 1200 series is the opne to look out for. I think they be now discountinued but a few places should still have them in stock.
Pm me if you want me to email you some pics.