Graphic cards / chips traditionally only support 2 outputs through the DVI, HDMI and VGA port. Many graphic cards have all three connections, but they are only connection options. You can use any two of the three at the same time. Laptops typically only have the VGA and HDMI ports. The built-in screen counts as one of the two monitors the graphics chip / card can output to.
AMD's Eyefinity is a relatively recent technology which allows you to connect up to 3 monitors, but the graphics card itself must have a DisplayPort and it must be used for the 3rd monitor. nVidia have some cards that can also handle 3 displays using the DisplayPort. Since laptops (to the best of my knowledge) do not have DisplayPort, they cannot support 3 displays at once.
The graphics chip / card has something called a RAMDAC (Random Access Memory Digital-to-Analog Converter) which can only handle two video streams. Thus, prior to the release of AMD's Eyefinity and nVidia's competing tech, all graphic cards can only support two monitors. The DisplayPort interface has it's own RAMDAC which allows for the support of a 3rd monitor. There are also graphic cards with multiple RAMDACs capable of supporting more than 2 monitors before DisplayPort ever existed, but these were expensive graphic cards. I would guess if it were ever implemented in to a laptop it could potentially increase the cost by $75 - $100.