Triple output video card
Many cards offer 3 display ports, but most can only drive 2 displays at a time.
I would like to expand to 3 monitors.
For this, you'll want a ATI Radeon HD 5xxx.
Drive three displays simultaneously with independent resolutions, refresh rates, color controls, and video overlays.
The ATI Radeon HD 5670 would be the entry level card, price around $100.
The ATI Radeon HD 5750 costs around $135.
All ATI Radeon HD 5xxx support Eyefinity multi-display technology:
Matrox also specializes in multi-monitor video cards. The M9138 LP PCIe x16 seems a good fit.
The Matrox M9138 LP PCIe x16 triple graphics card renders pristine image quality on up to three DisplayPort monitors at resolutions up to 2560 x 1600 per output, for an exceptional multi-monitor user experience. With 1 GB of memory and advanced desktop management features, such as independent and stretched desktop modes, the M9138 drives business, industrial, and government applications with extraordinary performance. Its low-profile form factor makes it easy to integrate into a wide variety of systems. It offers multiple operating system support, and can be paired with another M-Series card for additional monitor support.
You may also be interested in one of their other products, the TripleHead2Go:
Matrox TripleHead2Go is an external multi-display upgrade that adds up to three monitors to your notebook or desktop computer. Ideal for professionals requiring more desktop space for maximum productivity increases and gamers wanting a totally immersive experience, this small box lets you run different applications on each display or view one application across up to three monitors. Available in DisplayPort, Digital, or Analog Editions.