Hi Phil, welcome to the forum.
I'm not 100% sure but i believe the largest disks fitted to the A3 are 288mm (they were this size on my old 2002 A3 TDI 130 anyway).
You can replace these with the 312mm disks from the S3 easily using your existing calipers but you will need the caliper carriers from an S3 to replace your existing ones.
The R32 set up would also be another route (as long as it's a MK4 R32 kit) - i'm not sure of the size of the disks on the R32 but i'd assume you'd need the calipers, carriers & disks to do the conversion and that would probably cost a fair bit.
The other option is to replace the brakes with the brembo kit used on the old shape seat leon cupra R, these are 4 pot calipers and can handle pretty much anything you can throw at them, there's lots of posts on these so you might want to try a search for more information.
Good luck (and get some pics up of your car as well)
Mike