Did the fronts on my A4, brembo discs and OEM pads, £90 form GSF fitted up within an hour. Dare say the rears would cost a little less and about the same time to install. The wind back tool isn't essential for fitting the rear pads, but does make it easier.
I'm no Audi engineer, but I wished I charged their hourly rate.
How old is the A3 and what miles does it have, just wondering why it would need rear discs as they hardly ever wear out.