Make sure you have Measurement and Automation Explorer (MAX) and NI-VISA drivers installed. These should all be located on the device drivers CD.
If VISA is properly installed, you should see COM1, and whatever Com ports you have installed in your system, both in MAX and in a VISA resource constant or control. I'm not sure what you mean by comport 3F8. That sounds like one of the old ways of referring to serial ports by way of their hardware address. Its been years since I've seen that designation, I don't think it was information I ever really needed to know then, and I'm sure I've never seen it used in Labview.