From the above form, we can see that their function, firmware and software are maily different between BMW ICOM A,BMW ICOM A2 and BMW ICOM A3.

Conclusion：

1.The newest version ICOM A2 works better in cooling,runs faster and work more
  stable, not easier broken than older ICOM.
2.ICOM A3 can program all units for all BMW series expect media system.
3.ICOM A3 work more stable and has stronger compatibility than ICOM A2,A1.