Only thing I had to do was swap the seatbelt stalks, due to plug, otherwise bolt straight in, as I think the A5 ones do too, not sure of A3 though.
But have heard of A4 ones fitting in 9n polo, but unchecked personally, if that's possible then maybe A1, polo fabia etc seats would too?
We were in a scrapyard and me and my lad were getting bits for his polo, And my wife went for a look around, she came back with a big smile saying she had found some nice seats for the car, went to look and there was the A6 with full black leather, and behind it was an a4 b6 avant with just the rear seat left in matching black leather. £50 and 10 bolts later we had them in the back of the car!