I'm currently running 3 23" monitors from a single ATI HD5850 graphics card. There is a bit of a caveat, however. For EyeFinity to work, one of the displays must be through the DisplayPort output on the card. To achieve this, I am using a passive DisplayPort to DVI adapter, which is ideal for displays under 2560x1440 maximum resolution. For the higher resolution dislays, you need an active DP to DVI adapter.
So assuming that you have three displays capable of taking a DVI input and they aren't 30" screens, I would suggest an ATI card and a DisplayPort adapter.