Which bit of the grille? The surround pops off but the grille itself is a bumper off job.
Rough guide - S3 Bumper removal ( I pressume the A3 is similar):
1. Open the bonnet. There are 4 black plastic plugs, poke their centres through with a small driver and try to catch them. The plugs then lift out.
2. There are about 3 or 4 screws in each of the inner front arch which need removing, they have a torx type head. Only remove the screws in the bumper vicinity.
3. There are two larger bolt/screws on the bottom of the bumper about 12" each way of the centre and two screws in the bottom at each end of the bumper.
4. The bumper then just pulls off relatively easily. If it doesn't come easily, you've probably missed a screw.
5. Be sure to have a blanket or similar to rest the bumper on. You will see the grille clips from behind which just clip off.