I upgraded my debian desktop from wheezy to jessie today, and in doing so my triple monitor setup stopped working. This was originally a complicated thing to setup, and I had originally done it with a very specific xorg.conf file using Xinerama and some LeftOf RightOf screen setups. This didn't work after the upgrade, and X just launched into a black screen, and the Xorg.0.log just ended with a weird message about "dropping master: -22(invalid argument)".
So anyways I was messing around with the xorg.conf without any luck, and finally stumbled upon some useful information searching for a solution.
It turns out you can now just configure this whole thing with xrandr. In my case I was able to just run:
xrandr --setprovideroutputsource 1 0
This enables the second video card. And then I used a program (apt-get'able of course) called arandr which let me activate the third monitor on the second video card, and drag and drop the monitors into the correct physical layout location. arandr allows you to save a script that just runs the xrandr command needed. I had to add the bit from above to give me the following:
xrandr --setprovideroutputsource 1 0 xrandr --output VGA-1 --mode 1440x900 --pos 2880x0 --rotate normal --output HDMI-1 --off --output HDMI-1-2 --off --output DVI-I-1-2 --off --output VGA-1-2 --mode 1440x900 --pos 0x0 --rotate normal --output DVI-I-1 --mode 1440x900 --pos 1440x0 --rotate normal
Put that into your .Xsession or whatever you run on startup, and your monitors will get setup correctly.