As previously mentioned, Device Manager will probably give you a clue.
Open Device Manager and scroll down to "Ports (COM & LPT)"
Expand the Ports option. You should see a list of all active COM ports.
If COM3 is already in use then set ACL to use the other listed COM port.
The COM port number can be changed but be sure you change it to an unused COM port number.
Highlight the COM port you might need to change by doing one left click.
Once highlighted, right click and then select Properties.
Click on the Port Settings tab.
Click on the Advanced box.
A the bottom of the Advanced Setting window is a COM Port Number box. Left click on that box and then select an unused port number.
Click OK, then OK again.
You have now reassigned your COM port number.