The A2 is 5 x 100 PCD and 57.1 Bore IIRC, which is the same as the A3. The only thing I'm not sure about is the offset on the A2 vs the A3. OEM A2 wheels were 40 to 50 offset, and the A3 anywhere from 30 to 45 depending on model etc. (All this info on the Rochford Tyres website by the way.)
IIRC, the higher the offset number, the further in the arches the wheel will sit. So, at worst, you might need some spacers to fill the arches properly on an A3.
Best ask the seller though, there's still time!!