Hi Jaxxon, thanks for the hints.
The config is actually generated by nvidia-xconfig, nothing changed manually, but I'll try your hints anyway.
Does anyone know if it's possible that the cards cannot work together? Could it help to try two similar cards?
What really bothers me is that there is no error/warning, it's just ignoring the third monitor whatever I do. It even disappears in the nvidia-xconfig every time. Strange strange...
Damn, I'm working with one black monitor on my desk for quite a month now. My productivity is only two-thirds right now