Got this info off the TEX. Its not an A3 but the engine cover is the same...
1. Unclip the two metal clips.
2. Unclip the MAF sensor and pull out of the way.
3. Undo the air intake or undo the clip right behind the box area.
X. Pull. Cuss. Repeat. After a few attempts, pull harder and wiggle cover out. But don't pull to hard or in the wrong place, you can crack the cover!
These are the mount points (4 of them) on the engine. If there are any rubber pieces on the posts, pull them off and put them in the cover before re-assy.
And here's the underside of the cover showing the mount points
Fitting your new air filter
1) Turn cover upside down (place on something soft, as to not scratch the top).
2) Restore mounting bushings if you haven't done so yet.
3) Depending on how you took off the cover, remove the intake hose and the clamp.
4) Loosen 8 screws (circled in green). NO NEED TO REMOVE!
5) Separate cover halves from the front. Slowly, as there are hinges at the rear (by heatshield).
6) With pieces apart, change air filter.
When putting pieces back together, start with the hinge, and hold the air filter--so it doesn't fall out. Make sure screws are still pulled out, or cover won't seal. Follow steps in reverse to restore cover back to service.
All above thanks to Syntrix and
Chuibacca from the TEX.