Most video cards have multiple inputs in the card, maybe two VGA ports if it is an older card, or 2 DVI ports; or in whatever order. Some may have HDMI as well. You just need to plug in more than one monitor. If you want more monitors than the number of ports your video card has, you must buy another video card. You can buy a cheap video card to support these additional monitors. Just add that new video card in an open slot on your motherboard.
If you don't have an open slot, you may be out of luck. You could look at USB Video input dongles. They go for around 80-100 bucks each on various sites. However, most of them cannot support that high of a resolution.