Welly I had exactly the same problem and started a thread about it.
They are not the same. The S3 and TT are but the A3 1.8T are not. The S3 and TT have liquid inside them for better damping and are slightly stronger, they also have a heat sink effect on top of them for the liquid inside. You will notice what I mean if you can get a picture of a A3 or Seat Leon gearbox mount they are very slightly different visually. They will fit but not as strong so wouldnt advice you to do so.
I got a TT gearbox mount and was wondering why are A3 1.8T so much cheaer but that is why so only buy one from a TT or S3.