3 displays with NVIDIA native drivers

With a fresh install of Ubuntu 18.04 and the Nouveau drivers I was able to get 3 monitors working perfectly with 2 monitors attached via DVI to a Geforce 680 and a 3rd monitor attached to a Geforce 980-TI.

However, this is not quite a suitable configuration. I need CUDA so I need the to install the propriety drivers from NVIDIA.

After doing so, only two monitors are working. However, using nvidia-settings I was able to enable the third monitor attached to the 980-TI by creating a new X Screen 1 and saving the xorg.conf file to /etc/X11/xorg.conf

But after rebooting the 3rd monitor is simply black despite nvidia settings telling me it should be on. And the new Devices => Displays panel in the Ubuntu Settings manager shows only two monitors. With the Nouveau drivers installed three displays were shown here.

My xorg.conf configured with nvidia-settings contains:

# nvidia-settings: X configuration file generated by nvidia-settings
# nvidia-settings:  version 390.42  (buildd@lcy01-amd64-029)  Thu Mar 22 17:34:03 UTC 2018

# nvidia-xconfig: X configuration file generated by nvidia-xconfig
# nvidia-xconfig:  version 390.48  (buildmeister@swio-display-x86-rhel47-07)  Thu Mar 22 01:07:32 PDT 2018

Section "ServerLayout"
    Identifier     "Layout0"
    Screen      0  "Screen0" 0 0
    Screen      1  "Screen1" 3840 0
    InputDevice    "Keyboard0" "CoreKeyboard"
    InputDevice    "Mouse0" "CorePointer"
    Option         "Xinerama" "0"
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     "Monitor0"
    VendorName     "Unknown"
    ModelName      "DELL U2412M"
    HorizSync       30.0 - 83.0
    VertRefresh     50.0 - 61.0
    Option         "DPMS"
EndSection

Section "Monitor"
    Identifier     "Monitor1"
    VendorName     "Unknown"
    ModelName      "DELL U2412M"
    HorizSync       30.0 - 83.0
    VertRefresh     50.0 - 61.0
EndSection

Section "Device"
    Identifier     "Device0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce GTX 680"
    BusID          "PCI:4:0:0"
EndSection

Section "Device"
    Identifier     "Device1"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce GTX 980 Ti"
    BusID          "PCI:6:0:0"
EndSection

Section "Screen"

# Removed Option "nvidiaXineramaInfoOrder" "DFP-3"
# Removed Option "metamodes" "DVI-D-0: nvidia-auto-select +0+0, DVI-I-1: nvidia-auto-select +1920+0"
    Identifier     "Screen0"
    Device         "Device0"
    Monitor        "Monitor0"
    DefaultDepth    24
    Option         "Stereo" "0"
    Option         "nvidiaXineramaInfoOrder" "DFP-0"
    Option         "metamodes" "DVI-D-0: nvidia-auto-select +1920+0, DVI-I-1: nvidia-auto-select +0+0"
    Option         "SLI" "Off"
    Option         "MultiGPU" "Off"
    Option         "BaseMosaic" "off"
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection

Section "Screen"
    Identifier     "Screen1"
    Device         "Device1"
    Monitor        "Monitor1"
    DefaultDepth    24
    Option         "Stereo" "0"
    Option         "metamodes" "nvidia-auto-select +0+0 {AllowGSYNC=Off}"
    Option         "SLI" "Off"
    Option         "MultiGPU" "Off"
    Option         "BaseMosaic" "off"
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection

However, something stands out. Shouldn't there be 3 screen sections and another Monitor section? How do I get this to work?