Easy enough, but you need the right Graphics card/s.
I have two screens permanently running on mine, but could have three, as my card has 2 hdmi outputs and one VGA one.....if I plug a VGA monitor in to the third output, (or used a VGA to HDMI converter) it runs a third screen.
The other way of doing it is to get a second card (most cards these days have two outputs at minimum) but you'd need to make sure you had the correct slot available on your motherboard.......most MOBOS only have one dedicated Graphics slot (AGP older stylee, PCI express newer stylee) but you can get PCI graphics cards that will fit into one of the other, standard PCI slots (used for most cards, not just dedicated graphics cards)
Have a look in your control panel>system>hardware and see what is fitted, and also tell us what motherboard you have
EDIT:
Setting them up can be hard to accomplish unless you have both cards by the same manufacturer.......nVidia cards come with a nice bit of software that helps a lot and identifies the cards and monitors well, allows you to shuffle which display goes where and whether you want the main windows screen on the center one, the left or right one, or what orientation you want them in....You can have one screen set at 90 degrees to give you a portrait layout (for full A4 document display for example) or yopu can have the same display repeated on all three screens, or different display, or the desktop spread across 2 or three screens.....I've never tried that with different makes of cards, but I can imagine it being a right royal pain in the arse !