add 3rd monitor?

[AndreT]

I've hooked up an additional monitor to my Sony VAIO laptop under Windows 7 thru the serial port. How can I add a 3rd monitor? This laptop has another HDMI port but it's covered partially by the serial port connector. Sony Technical Support recommended external graphic card. Any recommendations? Websites to buy? I don't think I'll have more than 4 monitors.

Thanks to your help in advance.

 

[AndreT]

Checked around on External Graphic Card at Amazon and found a few models. All under $65 thru USB 2.0. Maybe USB 3.0 is faster?