DON'T DO IT - YOU COULD KILL YOURSELF!
Seriously, I've modded or tinkered with most parts of the A2, including fitting a TT wheel and airbag (new type) but modifying an airbag is not something I would contemplate.
The 4 spoke bag will not fit a 3 spoke wheel, even after modification, so save yourself a lot of aggro and find a proper bag in the first place.
Cheers,
Mike