Hey, welcome! Running a staggered set up is pretty common (Wider wheels in the rear than in the front). The wheels from the A3 will not fit the Boxster unfortunately. Your bolt pattern on the Audi is 5x112 and the majority of Porsche's bolt patterns are 5x130. You can however run adapters to make them fit although I wouldn't recommend it. It puts more stress on the hub/wheel bearing. I wouldn't track the car with adapters but they should be more than okay for city driving.
Now how about some pictures of your car?
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