OK - as far as I know, standard for an S3 is ET 54, so these wheels with an offset of
ET 42 means that these wheels will be 12mm closer to your arch.
I think that is right - though I'm sure someone who is more familier with this stuff will correct me.
If I am right, you should be fine, as it's just the equivalent of having 12mm spacers, which a lot of people fit.
You may get some rubbing on the rear, but it will be just a screw in the inside of the rear arch - lot;s have replaced this with a flatter screw to cure.
Hope that helps.
JK