i bought a third monitor yesterday and after hours of frustration, trial and error and a lot of caffeine, I finally got my triple-head setup working.
I'm using 2x nVidia GT630 and 3 fullHD 22" screens (and archlinux).
Because of the missing multiple-GPU support in XRandR, I had to use xinerama.
Now X11 recognizes all three monitors perfectly, but KDE doesn't.

Here's what my desktop looks: http://0wx.de/993c65dd0dfe67ddcdc26dfb36c8733c.png (linked, not embedded, because it's a 5760x1080 screenshot)

Moving windows to the right screen is working without any problem.
But the plasma desktop is working on the left and the middle screen only.
I can't set a wallpaper, I cant setup a panel or a plasmoid, i can't even right click at an empty space on the right screen and get a menu.
If i toggle maximize on a window on the right screen, it just maximizes on the middle screen. Also if i play a video in Kaffeine, Dragon Player or VLC and toggle fullscreen, fullscreen enables on the middle screen or the left screen only. If i toggle fullscreen on a window on the right monitor, it just shows in fullscreen on the middle one.

As you can see in the screenshot, the monitor setup in the KDE systemsettings dialog isn't working, because of the missing XRandR i mentioned before. (It's in german on my screenshot, but it should be obvious, that it's not working.)

I also tried removing my ~/.kde4 folder to see if the third monitor would be recognized automatically, after it was set up for 2 monitors before, but this didn't work.

I'm wondering, if theres any config file in ~/.kde4/share/config, where i could set up my third monitor, so it's usable by plasma-desktop.

Moved to the KWin forum to get an answer concerning the maximisation issue. I suspect this may be caused by the monitors being powered by different graphics cards however.