Bit of a minefield, this, as Win2K doesn't suport the old physical serial ports directly. Notwithstanding, an elementary first step is to make sure that the BIOS is set up correctly.
You don't say what type of interface it is you're trying to connect to Com3. Is it a USB modem? if so it is likely that the installation disk included a routine for installing a USB hub.
So for starters I'd go back to the modem installation disk and see what is there.
But it's late now. Maybe more tomorrow if needed.