Simply plugging three monitors into the outputs of two cards is fine for displaying a massive desktop, if that's all you need to do. If you're planning on running games fullscreen, most of them can be very funny about running on multiple monitors using the native windows drivers.
SoftTH is probably your best bet:
http://www.kegetys.net/SoftTH/
This lets you use a second graphics card as the input to the third monitor and emulates it as one large display - the second card doesn't have to be anything special, as it's not used for processing at all, just for outputting a signal.
You can also use Matrox' TripleHead2Go into which you plug your three monitors and windows sees them as one super-wide monitor. You only need one graphics card for this, as it only uses one output from the PC. You need monitors of equal resolution though so it's not the best option for you.
Be aware, (whichever method you use) that if you're running XP, there's a limitation of DirectX 9 which means that the maxium hardware-rendered image you can display is 4096 pixels wide - any wider and it'll either crash or drop to software rendering. DX10 is supposed to up this to 8192 pixels wide, but I have no idea if it works.
