You shouldn't have to change anything. If the BT Virtual COM port on the PC is COM3 then that is fine.
1. Make sure that the BT serial port service on the PC is enabled.
2. Make sure that the BT radio is active on the PC.
3. Goto BT Manager on the PPC search for new BT devices.
4. Locate and setup the connection for the serial port on the PC.
5. On the desktop configure Activesync to use COM3 for connection properties.
6. Double tap the activesync BT icon. You should be able to sync.
For more info take a look at my BT guide found at
http://www.aximsite.com/bluetooth/index.htm.