3 Screens, two Dual-Head graphic cards

[Husse]

This is an odd setup (no offence meant ) and it's the first time I bump into it
You did not tell us which Mint you have but regardless of that I'd recommend that you let envy uninstall the driver and then install it again - envy is good at that
In Elyssa you find Envy in the menu in Felicia you have to start it from the terminal or a launcher (Alt+F2)
The command is envyng -t
Also xinerama may help you (you find it in synaptic)