I run tripple monitors mainly for development, the cheapest option for myself was buying another graphics card to use with existing card, you really need a motherboard with an extra pci-e slot, else finding another DVI pci graphics card may be difficult.
My existing card was an old 7800GTX, and the new card was a GTX 260. I put the 260 as my primary card, and the 78000GTX as secondary.
If your going down this path it's a little easier if you have cards that use the same drivers, both those above do. Also if it's 2d work any cheap pci-e card from ebay will be fine, I almost purchased a 7100GS for £12, but got a good deal on the GTX 260.
The only downside with above solution is I can't run games over three monitors, 2 yes but not 3. You require a solution above if it's tripple monitors for gaming.