An ATI card that supports 3 monitors (needs to have a displayport connector and you need an "active" displayport adapter or a monitor with a displayport connector) should work. But don't tell anyone I recommended an ATI card for linux.
ard0r, if you were to recommend an ATI card which had three display ports, what would it be? And, as a guy who has used nvidia with Linux for the past, oh, 14 years or so, can you tell me why you seem reluctant to recommend an ATI (AMD)? Am I inviting trouble by purchasing one?
I have a 5870 eyefinity 6 that has 6 display ports, but those are no longer made and expensive (and overkill).
I found a $60 Radeon HD 6450 card made by Sapphire on Newegg (N82E16814102960) that has two dvi and one displayport, that should work for three monitors. There are other options as well of course, just make sure the cards are advertised as supporting eyefinity and they have at least one displayport adapter. I've had success with an Accel active displayport to DVI adapters.
Not having compositing on the linux desktop these days means you're a second class season. Both Unity and Gnome3 require it to function...if you don't have it, you have to fall back to Unity2D (incomplete user experience) or Gnome 2 (no longer supported. KDE actually does the best of the bunch when it comes to Xinerama so far, but I really prefer Unity or Gnome3.
I've read that if you buy two similar Nvidia cards (same GPUs), you can get compositing across three monitors, but I've not spoken with anyone who can verify. If anyone reading this can, please provide your vid card model.
Do you know if this Radeon HD 6450 will support compositing across three monitors? $60 is cheap to me to get this.
Thanks!
Anyway, I hope this helps!