I have an HP docking station that has two dvi ports. I plugged in two monitors into the dvi ports, however now I can't use my laptop builtin screen as a third monitor... Is this a graphics card limitation? Or some kind of Windows 7 limitation that I could fix with extra software.
Without specific models it's hard to tell. But if it is live the HP Probooks we got in last year it is likely you can only use either two external monitors OR one external monitor and the laptop screen.
As far as I can tell it is a hardware limitations.
If you MUST have three or more monitors you may want to look at an external video card, though I'm not sure if there is anything commercially available (a quick Google shows a lot of DIY eGPU ideas.)