Your RIM ports come from it (Blackberry is made by RIM).
The other ports Windows says are in use may or might not be. They are reserved but "in use" is misleading. Do you use a USB serial adapter to connect the AIS? If nothing other than your Blackberry is connected and it uses Com 5 & 6 you can reassign your AIS to Com 3. Worse case is you'll change it back if something stops working.
Steve I think you've got enough info to work this out on your own. I'll send my bill (grin).