A COM3 (or Communications 3) port is a communications channel on the computer that is used to transmit data from hardware devices connected to the PC to the processor. While most computers will only utilize the COM1 and COM2 ports, adding the COM3 port to your computer can be necessary where you have additional devices on the machine that are using the communications channel.
Right click "My Computer," select "Properties," and select "Hardware."
Click on "Add New Hardware" and select "I have already connected the hardware."
Click "Add a new hardware device" and click "Install hardware I manually select."
Select "Ports (COM and LPT)," press "Next," and press "Communications Port." This will install an additional COM port (e.g. installing COM3 if 1 and 2 are already installed or installing COM2 if only COM1 is installed).
Repeat the procedure if necessary to install an additional COM port.