Xorg conf for 3 monitors and separate xscreens for each monitor.

xorg.conf

# nvidia-settings: X configuration file generated by nvidia-settings
# nvidia-settings:  version 396.24  (builduser@felixonmars2)  Wed May  9 20:37:28 CST 2018

# nvidia-xconfig: X configuration file generated by nvidia-xconfig
# nvidia-xconfig:  version 390.67  (buildmeister@swio-display-x64-rhel04-18)  Fri Jun  1 04:26:22 PDT 2018

Section "ServerLayout"

# Removed Option "Xinerama" "1"
    Identifier     "Layout0"
    Screen      0  "Screen0" 1680 0
    Screen      1  "Screen1" 0 15
    Screen      2  "Screen2" RightOf "Screen0"
    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      "Ancor Communications Inc ASUS VS228"
    HorizSync       24.0 - 83.0
    VertRefresh     50.0 - 75.0
    Option         "DPMS"
EndSection

Section "Monitor"
    Identifier     "Monitor1"
    VendorName     "Unknown"
    ModelName      "Gateway FPD2275W"
    HorizSync       31.0 - 83.0
    VertRefresh     56.0 - 76.0
EndSection

Section "Monitor"
    Identifier     "Monitor2"
    VendorName     "Unknown"
    ModelName      "Ancor Communications Inc ASUS VS228"
    HorizSync       24.0 - 83.0
    VertRefresh     50.0 - 75.0
EndSection

Section "Device"
    Identifier     "Device0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce GTX 550 Ti"
    BusID          "PCI:2:0:0"
    Screen          0
EndSection

Section "Device"
    Identifier     "Device1"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce GTX 970"
    BusID          "PCI:3:0:0"
EndSection

Section "Device"
    Identifier     "Device2"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce GTX 550 Ti"
    BusID          "PCI:2:0:0"
    Screen          1
EndSection

Section "Screen"

# Removed Option "MetaModes" "GPU-1.DVI-I-0: 1680x1050+0+0, GPU-0.DVI-I-1: 1920x1080+1680+0, GPU-1.DVI-I-2: 1920x1080+3600+0"
# Removed Option "BaseMosaic" "True"
# Removed Option "metamodes" "DVI-I-2: 1920x1080 +1920+0, DVI-I-3: 1920x1080 +0+0"
# Removed Option "metamodes" "DVI-I-3: 1920x1080 +0+0"
# Removed Option "metamodes" "DVI-I-3: 1920x1080 +0+0, DVI-I-2: nvidia-auto-select +1920+0"
    Identifier     "Screen0"
    Device         "Device0"
    Monitor        "Monitor0"
    DefaultDepth    24
    Option         "Stereo" "0"
    Option         "nvidiaXineramaInfoOrder" "DFP-2"
    Option         "metamodes" "DVI-I-3: 1920x1080 +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" "1680x1050 +0+0 {AllowGSYNC=Off}"
    Option         "SLI" "Off"
    Option         "MultiGPU" "Off"
    Option         "BaseMosaic" "off"
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection

Section "Screen"
# Removed Option "metamodes" "DVI-I-2: 1920x1080 +0+0 {AllowGSYNC=Off}"
    Identifier     "Screen2"
    Device         "Device2"
    Monitor        "Monitor2"
    DefaultDepth    24
    Option         "Stereo" "0"
    Option         "metamodes" "DVI-I-2: nvidia-auto-select +0+0 {AllowGSYNC=Off}"
    Option         "SLI" "Off"
    Option         "MultiGPU" "Off"
    Option         "BaseMosaic" "off"
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection