how to configure xorg.conf for 3 monitors

I recently installed a Dell machine with dual nVidia video cards.
I ran system-config-display to configure xorg.conf and it has worked
fine for 2 of the 3 monitors. I didn't see anything in
system-config-display that would enable me to configure xorg.conf for
3 monitors. Did I miss something?? Is there a way to modify xorg.conf
to have 3 monitors??

Thanks in advance,
-Mark

you need a Device, Monitor, and Screen section in your xorg.conf for each monitor. Then you need to put all 3 Screens in your ServerLayout section.

Section "ServerLayout"   
        Identifier      "threescreens"
        Screen          "screen1" 0 0
        Screen          "screen2" RightOf "screen1"
        Screen          "screen3" RightOf "screen2" 0 0
        InputDevice     "keyboard" "CoreKeyboard"
        InputDevice     "mouse" "CorePointer"
EndSection

The Device sections need BusID lines to specify the video card they refer to. You may need to use the Nvidia blob's options "ConnectedMonitor" or "UseDisplayDevice" for the card with 2 monitors, but you'll have to read up on that yourself (Nvidia's README docs cover allsorts). The README also has more info on multi-monitor setups.