Hello Kev,
Unless things have changed with the A3, they don’t come with pre-drilled fixing points for fitting the front to back roof rails, on which the roof bars sit on/connect to, so if you haven’t specified roof rails within your build, then effectively you wont practically be able to retro fit them, without drilling the roof!
If you are talking about fitting the clamp-under- door-frame type roof cross bars then yours might fit. Personally, I’m not a fan of this type of fixing, but….
Kind regards,
Gareth.