Two Radeon 5000 series cards, assuming you get them with display port. ATI cards can run 3 monitors if one of the outputs is display port. You will have to use at least one monitor with display port or an active converter though. The cheapest one that will support 3 monitors is
It's a dog if you want to use any kind of 3d acceleration on it though. If you don't have monitors with display port, or you don't want to pay for the converter, then you can just use a board that has 3-PCI-E slots and any modern low end GPU. Just about every modern GPU can drive at least two displays at the same time.