I have seen that you didn't enabled xinerama in your xorg.conf.
This is ok for a twinview setup, because the nvidia xinerama extension
is enabled by default, providing the appropriate information
(unless disabled with NoTwinViewXineramaInfo).
If you add another screen with a second card, then TwinViewXineramaInfo
is disabled - AFAIK, it is currently not possible to have Xinerama only for a
subset of screens.
To truely form three separate screens, you need to define two
screen for the first card as described in chapter P ("Configuring
Multiple X Screens on One Card").
If your WM is not capable of handling separate Screens then you
need to apply the option "Xinerama" "on".
If you like to use all three displays with xinerama, but you are
not happy with the XineramaInfo because of TwinView, then
you may like to read the following post: