The Radeon 5000 series cards support having more than one monitor. Using a daisy chain display port configuration, you can have up to six. That's eye-infinity, but currently it does have a few kinks to work out. You can use a Radeon 5970 to power more than one screen while gaming, but currently you can't do so with two 5870s in crossfire.
Anyway, if you want to power 3 monitors for now just get a Radeon 5870. Read reviews of the 5870 if you want to know more about eyeinfinity.