This is more a personal note than a real post. I just struggled for 4 hours to have my 2+1 screen to work just fine.

Here’s my secret :

xrandr --fb 4160x1024 --output LVDS-1 --mode 1600x900 --pos 2560x120 
xrandr --fb 4160x1024 --output VGA-1 --mode 1280x1024 --pos 1280x0 
xrandr --fb 4160x1024 --output DP-3 --mode 1280x1024 --pos 0x0