there have been a couple people that have done this. The biggest problems will be:
1. There are no dash kits for the 3 just yet. You would have to do some type of custom panel so you don't see the innards of the deck. If you just put a single DIN unit in, there will also be a huge amount of custom panel making the head unit look... well, odd to say the least.
2. Unless you do a little custom work, the steering wheel controls won't work.
3. The nice factory display is close to worthless if you put a new head unit in.