Have you already managed to have X working with each monitor at a single time?
I know how to have triple head using 2 ATI cards using xinerama (no direct rendering), but I guess eyefinity enabled cards can do triple head with direct rendering. At least, that's what I see in this video: http://www.youtube.com/watch?v=N6Vf8R_gOec.
Were those crashes before you turned off Xinerama or after ?
Before. After I turned it off, everything was working fine. Found a thread that mentioned that Xinerama with more than 3 monitors will crash X (with nvidia as well).
I now purged the fglrx driver and switched back to the open source driver. Found a xorg.conf online that someone got working with XRandR and 3 monitors. Just changed it to fit my setup and I am up and running with 3 monitors and the open source driver.
Thanks. BTW if you run all 3 displays on one card you should be able to get Compiz etc..
I haven't tried it myself but it should all fit together pretty well. The Eyefinity GPUs let you take X up to 6 displays (on a single GPU, assuming the card has enough of the right connectors) without having to deal with multi-card issues.