You need to buy a PCI Express graphics card which allows you to run three monitors off it. You install it in to a spare PCI Express slot on the motherboard (if your motherboard has a PCI Express slot, older motherboards don't have one).
When you install a PCI or PCI Express graphics card you won't be able to use the on-board graphics adapter at all because the BIOS disables it automatically to avoid a resource conflict. That's why you need a card which supports three monitors.