TwinView works for two monitors on one graphics card, but once you get a second card you'll need xinerama, ergo no 3d support.
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.
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.