Is there a reason you're considering 3 monitors vs 2 (larger) ones?
Not implying that 2 monitors is better than 3, just making sure you've considered that possibility.
First, check this out: http://en.wikipedia.org/wiki/Multi-monitor#Multi-display_set...
You definitely want a good video card, but you can pick up an nVidia 660 or Radeon 6770 for under $200. You'll need that should you ever do anything, like games, that requires a dedicated graphics card.
I recommend you then attach that to a Matrox TripleHead2Go: http://www.matrox.com/graphics/en/products/gxm/th2go/display...
This is what I use to mount my monitors: http://www.google.com/products/catalog?q=multi+monitor+displ... (originally posted wrong one)
Enjoy! It's awesome!
Prefer a video card setup that can drive 3 monitors over the Matrox things.
The Matrox thing will make 2 or more monitors appear as one. Under Linux you can hack xinerama to deal with this. The Mac support makes it annoying to use. I haven't tried with windows but I imagine its ok there.
But get a modern card that can drive 3 monitors. It should be eaiser.
I've been running 3 24" screens for years now. But am thinking of going to one 27.