I'm currently using a NVIDIA GT218 (for one monitor) and a GT520 (for two other monitors) on my Arch Linux system. Although Xinerama support is nearly unusable with this setup (most likely due to the different chipsets on the cards), you can do TwinView (for two monitors) and a separate X screen (for the third) or 3 separate xscreens. I just do 3 separate xscreens (I'm a gamer and don't like to see my quake client stretched across two of my monitors and my TV :) ). You can't move windows between monitors with Xinerama or TwinView, but that's a sacrifice I was willing to make for my configuration. I am able to do compositing across all three of my monitors with this configuration.

Anyway, I hope this helps!