cat /etc/X11/xorg.conf.d/40-monitor.conf
Section "Device"
  Identifier "Onboard Radeon"
  Driver     "radeon"
  Option     "Monitor-HDMI-0" "Right screen"
  BusID      "PCI:1:5:0"
EndSection
Section "Device"
  Identifier "PCI nVidia"
  Driver     "nvidia"
  BusID      'PCI:2:0:0"
  Option     "Monitor-DVI-0" "Left screen"
  Option     "Monitor-DVI-1" "Center screen"
EndSection
Section "Monitor"
  Identifier "Left screen"
EndSection
Section "Monitor"
  Identifier "Center screen"
  Option     "RightOf" "Left screen"
EndSection
Section "Monitor"
  Identifier "Right screen"
  Option     "RightOf" "Center screen"
EndSection