I tried with this problem for 2 years already. LOL, today, I install Ubuntu for I have to resolve it or I will back to stupid Windows 7.

First, I have 3 monitors. My graphic card is support dual ( ATI Radeon ), so I have no problem on extend those multi monitor on VGA and DVI.

The 3rd monitor is Displaylink USB. After installed everything required, when I reboot, the displaylink monitor show "Ubuntu ...." like logo / loading screen.

I go to System > Display , Detect monitor, it only show my 1st and 2nd, NO 3RD Displaylink.

I can move my mouse between those 1st & 2nd, but the 3rd is only show the Ubuntu Screen.

I press Ctrl+Alt+1, then screen switch to Displaylink USB 3RD monitor, but its "Terminal" not a desktop.

Then I press Ctrl+Alt+7 , the screen switch back to my 1st, 2nd, and the displaylink 3rd is witch back to Logo / Ubuntu again.

This is my /etc/X11/xorg.conf :

Section "ServerLayout"
    Identifier     "X.org Configured"
    Screen      0  "aticonfig-Screen[0]-0" 0 0
    Screen      1  "DisplayLinkScreen" Leftof "aticonfig-Screen[0]-0"
    InputDevice    "Mouse0" "CorePointer"
    InputDevice    "Keyboard0" "CoreKeyboard"
EndSection

Section "Files"
    ModulePath   "/usr/lib/xorg/modules"
    FontPath     "/usr/share/fonts/X11/misc"
    FontPath     "/usr/share/fonts/X11/cyrillic"
    FontPath     "/usr/share/fonts/X11/100dpi/:unscaled"
    FontPath     "/usr/share/fonts/X11/75dpi/:unscaled"
    FontPath     "/usr/share/fonts/X11/Type1"
    FontPath     "/usr/share/fonts/X11/100dpi"
    FontPath     "/usr/share/fonts/X11/75dpi"
    FontPath     "/var/lib/defoma/x-ttcidfont-conf.d/dirs/TrueType"
    FontPath     "built-ins"
EndSection

Section "Module"
    Load  "glx"
    Load  "dri2"
    Load  "dbe"
    Load  "dri"
    Load  "record"
    Load  "extmod"
EndSection

Section "InputDevice"
    Identifier  "Keyboard0"
    Driver      "kbd"
EndSection

Section "InputDevice"
    Identifier  "Mouse0"
    Driver      "mouse"
    Option      "Protocol" "auto"
    Option      "Device" "/dev/input/mice"
    Option      "ZAxisMapping" "4 5 6 7"
EndSection


Section "Monitor"
    Identifier   "aticonfig-Monitor[0]-0"
    Option      "VendorName" "ATI Proprietary Driver"
    Option      "ModelName" "Generic Autodetecting Monitor"
    Option      "DPMS" "true"
EndSection

Section "Monitor"
    Identifier   "DisplayLinkMonitor"
EndSection

Section "Monitor"
    Identifier   "0-DFP1"
    Option      "VendorName" "ATI Proprietary Driver"
    Option      "ModelName" "Generic Autodetecting Monitor"
    Option      "DPMS" "true"
    Option      "PreferredMode" "1680x1050"
    Option      "TargetRefresh" "60"
    Option      "Position" "0 0"
    Option      "Rotate" "normal"
    Option      "Disable" "false"
EndSection

Section "Monitor"
    Identifier   "0-CRT2"
    Option      "VendorName" "ATI Proprietary Driver"
    Option      "ModelName" "Generic Autodetecting Monitor"
    Option      "DPMS" "true"
    Option      "PreferredMode" "1920x1080"
    Option      "TargetRefresh" "60"
    Option      "Position" "1680 0"
    Option      "Rotate" "normal"
    Option      "Disable" "false"
EndSection

Section "Device"

        ### Available Driver options are:-
        ### Values: <i>: integer, <f>: float, <bool>: "True"/"False",
        ### <string>: "String", <freq>: "<f> Hz/kHz/MHz",
        ### <percent>: "<f>%"
        ### [arg]: arg optional
        #Option     "NoAccel"               # [<bool>]
        #Option     "SWcursor"              # [<bool>]
        #Option     "Dac6Bit"               # [<bool>]
        #Option     "Dac8Bit"               # [<bool>]
        #Option     "BusType"               # [<str>]
        #Option     "CPPIOMode"             # [<bool>]
        #Option     "CPusecTimeout"         # <i>
        #Option     "AGPMode"               # <i>
        #Option     "AGPFastWrite"          # [<bool>]
        #Option     "AGPSize"               # <i>
        #Option     "GARTSize"              # <i>
        #Option     "RingSize"              # <i>
        #Option     "BufferSize"            # <i>
        #Option     "EnableDepthMoves"      # [<bool>]
        #Option     "EnablePageFlip"        # [<bool>]
        #Option     "NoBackBuffer"          # [<bool>]
        #Option     "DMAForXv"              # [<bool>]
        #Option     "FBTexPercent"          # <i>
        #Option     "DepthBits"             # <i>
        #Option     "PCIAPERSize"           # <i>
        #Option     "AccelDFS"              # [<bool>]
        #Option     "IgnoreEDID"            # [<bool>]
        #Option     "CustomEDID"            # [<str>]
        #Option     "DisplayPriority"       # [<str>]
        #Option     "PanelSize"             # [<str>]
        #Option     "ForceMinDotClock"      # <freq>
        #Option     "ColorTiling"           # [<bool>]
        #Option     "VideoKey"              # <i>
        #Option     "RageTheatreCrystal"    # <i>
        #Option     "RageTheatreTunerPort"  # <i>
        #Option     "RageTheatreCompositePort"  # <i>
        #Option     "RageTheatreSVideoPort"     # <i>
        #Option     "TunerType"             # <i>
        #Option     "RageTheatreMicrocPath"     # <str>
        #Option     "RageTheatreMicrocType"     # <str>
        #Option     "ScalerWidth"           # <i>
        #Option     "RenderAccel"           # [<bool>]
        #Option     "SubPixelOrder"         # [<str>]
        #Option     "ClockGating"           # [<bool>]
        #Option     "VGAAccess"             # [<bool>]
        #Option     "ReverseDDC"            # [<bool>]
        #Option     "LVDSProbePLL"          # [<bool>]
        #Option     "AccelMethod"           # <str>
        #Option     "DRI"                   # [<bool>]
        #Option     "ConnectorTable"        # <str>
        #Option     "DefaultConnectorTable"     # [<bool>]
        #Option     "DefaultTMDSPLL"        # [<bool>]
        #Option     "TVDACLoadDetect"       # [<bool>]
        #Option     "ForceTVOut"            # [<bool>]
        #Option     "TVStandard"            # <str>
        #Option     "IgnoreLidStatus"       # [<bool>]
        #Option     "DefaultTVDACAdj"       # [<bool>]
        #Option     "Int10"                 # [<bool>]
        #Option     "EXAVSync"              # [<bool>]
        #Option     "ATOMTVOut"             # [<bool>]
        #Option     "R4xxATOM"              # [<bool>]
        #Option     "ForceLowPowerMode"     # [<bool>]
        #Option     "DynamicPM"             # [<bool>]
        #Option     "NewPLL"                # [<bool>]
        #Option     "ZaphodHeads"           # <str>
    Identifier  "Card0"
    Driver      "radeon"
    BusID       "PCI:5:0:0"
EndSection

Section "Device"

        ### Available Driver options are:-
        ### Values: <i>: integer, <f>: float, <bool>: "True"/"False",
        ### <string>: "String", <freq>: "<f> Hz/kHz/MHz",
        ### <percent>: "<f>%"
        ### [arg]: arg optional
        #Option     "ShadowFB"              # [<bool>]
        #Option     "Rotate"                # <str>
        #Option     "fbdev"                 # <str>
        #Option     "debug"                 # [<bool>]
    Identifier  "Card1"
    Driver      "fbdev"
    BusID       "PCI:5:0:0"
EndSection

Section "Device"

        ### Available Driver options are:-
        ### Values: <i>: integer, <f>: float, <bool>: "True"/"False",
        ### <string>: "String", <freq>: "<f> Hz/kHz/MHz",
        ### <percent>: "<f>%"
        ### [arg]: arg optional
        #Option     "ShadowFB"              # [<bool>]
        #Option     "DefaultRefresh"        # [<bool>]
        #Option     "ModeSetClearScreen"    # [<bool>]
    Identifier  "Card2"
    Driver      "vesa"
    BusID       "PCI:5:0:0"
EndSection

Section "Device"
    Identifier  "aticonfig-Device[0]-0"
    Driver      "fglrx"
    Option      "Monitor-DFP1" "0-DFP1"
    Option      "Monitor-CRT2" "0-CRT2"
    BusID       "PCI:5:0:0"
EndSection

Section "Device"
    Identifier  "DisplayLinkDevice"
    Driver      "displaylink"
    Option      "fbdev" "/dev/fb1"
    Option "ShadowFB" "off"
EndSection


Section "Screen"
    Identifier "aticonfig-Screen[0]-0"
    Device     "aticonfig-Device[0]-0"
    DefaultDepth     24
    SubSection "Display"
        Viewport   0 0
        Virtual   3600 1920
        Depth     24
    EndSubSection
EndSection

Section "Screen"
    Identifier "DisplayLinkScreen"
    Device     "DisplayLinkDevice"
    Monitor    "DisplayLinkMonitor"
    DefaultDepth     24
    SubSection "Display"        
        Depth     24
        Modes    "1920x1080"
    EndSubSection
EndSection