I have the Audi roofrack for my 3dr which is great. It only takes a couple of minutes to put on, and is solid as a rock. As with all roofracks you do have to be super-carefull when putting it on/taking it off, so as not to scratch the paintwork....
I know Thule also do one that's available from Halfords. It's about £40 cheaper than the Audi one I think (mine was £170), but I thought I'd play it safe and go for the Audi (had a Thule for my golf and it was fine though).
a few months before I ordered my A3 I got worried because the dealer I visited said that they had problems with the roofrack, but I ended up seeing one on someone's car a month or so later.