Sorry you weren't made to feel welcome. I'm not sure why the post was deleted, but you're here now, so here's big welcome from me and all the forum
The should fit. I say 'should', because while the rolling radius is closely maintained, and the wheels would only poke an extra 8mm over the 15s, the inner rim might be a little close to the inner arch and/or suspension strut.
Willtheyfit.com is a very handy tool that lets you enter two sets of wheel data (like a before and after), then calculates the physical difference between the two in terms of how the two sets of wheels sit in the arch. It even gives you a diagram as well as the various dimensions.
Here's what it says about your wheels:
http://www.willtheyfit.com/index.php?width=195&aspect=65&diameter=15&wheelwidth=6&offset=45&width2=225&aspect2=45&wheel_size=17&wheel_width=7-5&offset2=56Like I said, you can see that the only big difference is the inner side of the wheel & tyre, and therefore the question is whether you'll get clearance issues.
So, can you tell us more about the 17" wheels you want to fit? It would help a lot if you could tell us more about the car they came off. Do you know the year of the A3? You need to establish if the PCD (bolt pattern) is correct, first and foremost. 5x112 is what you're looking for. It should hopefully be stamped on the rear of the wheels.
Equally, if anyone else on the forum is familiar with 17" fitment on our cars, let us know.
Usually the offset I see associated with 7.5" rims on our cars is et51. Et56 is therefore 'only' 5mm closer to the inner arch and suspension, so my gut feel is that they'd be ok. But I'd rather we give you a definite yes or no based on fact.
You can fine tune your offsets with wheel spacers - and in doing so you'd certainly be able to get these wheels to fit - but for the sake of the question I'll assume that's not (yet) an option.
I hope that helps somewhat, and once again, apologies if you were made to feel unwelcome.