In device manager either delete the USB serial port (com 8) or go into that ports properties and reassign it to Com 3. Save any changes and reboot the computer. Again check the device manager for the cable as com 3. If all well set com 3 in VCDS. You may have to change the Baud rate if too many communication errors or VCDS fails to connect.
Unless you really are using some COM ports on computer, i'd have them disabled anyway in BIOS. So in my case there already would be a free COM probably available. Another option is to reassign one of smaller numbers to larger and then COM 8 to lower freed up one... COM 2 would probably be safest bet if done blindly.