If your motherboard has two full-size 16x PCI-Express slots, you can use two PCI-Express graphics cards to drive 3/4 monitors. Most chipsets will however reduce each slot to 8x speed. If you intend to run any sort of 3D application across all three monitors, this is your best bet.
If your motherboard only has one full size (16x) slot, then you will have to use something like the Matrox G550, which will fit in a smaller 1x PCI-Express slot. Alternatively you could use an old PCI graphics card. The Matrox solution would work well enough with 2D graphics, but don't expect to use either of these solutions for games.
