Mais de 2 monitores não são bem suportados pelas ferramentas GUI, na minha experiência. É possível configurar o terceiro monitor manualmente no xorg.conf, mas não haverá nenhum comportamento plug-and-play bacana. Você precisará de uma seção adicional Dispositivo, Tela e Monitor para ativar o terceiro monitor, depois adicionar a Tela à seção ServerLayout. Aqui está minha tela de trabalho xorg.conf
como exemplo:
Section "ServerLayout"
Identifier "Layout0"
Screen 0 "Screen1" 0 0
Screen 1 "Screen2" LeftOf "Screen1"
Screen 2 "Screen3" LeftOf "Screen2"
Screen 3 "Screen4" LeftOf "Screen3"
InputDevice "Keyboard0" "CoreKeyboard"
InputDevice "Mouse0" "CorePointer"
Option "Xinerama" "1"
EndSection
Section "Files"
EndSection
Section "InputDevice"
# generated from default
Identifier "Mouse0"
Driver "mouse"
Option "Protocol" "auto"
Option "Device" "/dev/psaux"
Option "Emulate3Buttons" "no"
Option "ZAxisMapping" "4 5"
EndSection
Section "InputDevice"
# generated from default
Identifier "Keyboard0"
Driver "kbd"
EndSection
Section "Monitor"
Identifier "Monitor1"
VendorName "Samsung"
ModelName "Samsung ??"
HorizSync 30.0 - 81.0
VertRefresh 56.0 - 60.0
EndSection
Section "Monitor"
Identifier "Monitor2"
VendorName "Unknown"
ModelName "Samsung SyncMaster"
HorizSync 30.0 - 81.0
VertRefresh 56.0 - 75.0
EndSection
Section "Monitor"
Identifier "Monitor3"
VendorName "Unknown"
ModelName "Samsung SyncMaster"
HorizSync 30.0 - 81.0
VertRefresh 56.0 - 60.0
Option "DPMS"
EndSection
Section "Monitor"
Identifier "Monitor4"
VendorName "Unknown"
ModelName "WestingHouse"
HorizSync 30.0 - 81.0
VertRefresh 56.0 - 60.0
Option "DPMS"
EndSection
Section "Device"
Identifier "Device1"
Driver "nvidia"
VendorName "NVIDIA Corporation"
BusID "PCI:1:0:0"
Screen 0
EndSection
Section "Device"
Identifier "Device2"
Driver "nvidia"
VendorName "NVIDIA Corporation"
#BoardName "GeForce 8800 GTS"
BusID "PCI:1:0:0"
Screen 1
EndSection
Section "Device"
Identifier "Device3"
Driver "nvidia"
VendorName "NVIDIA Corporation"
#BoardName "GeForce 9500 GT"
BusID "PCI:2:0:0"
Screen 0
EndSection
Section "Device"
Identifier "Device4"
Driver "nvidia"
VendorName "NVIDIA Corporation"
#BoardName "GeForce 9500 GT"
BusID "PCI:2:0:0"
Screen 1
EndSection
Section "Screen"
Identifier "Screen1"
Device "Device1"
Monitor "Monitor1"
DefaultDepth 24
EndSection
Section "Screen"
Identifier "Screen2"
Device "Device2"
Monitor "Monitor2"
DefaultDepth 24
SubSection "Display"
Depth 24
EndSubSection
EndSection
Section "Screen"
Identifier "Screen3"
Device "Device3"
Monitor "Monitor3"
DefaultDepth 24
SubSection "Display"
Depth 24
EndSubSection
EndSection
Section "Screen"
Identifier "Screen4"
Device "Device4"
Monitor "Monitor4"
DefaultDepth 24
SubSection "Display"
Depth 24
EndSubSection
EndSection