First, I would like to note that the answer I am about to provide is based on answers I gave for the questions here and here.

Second, take note of the information found on Intel's site. It suggests that it can work, but everything I have read thus far leads me to believe that three monitors will not work in the real world. That being said, Let's dive in and explore the possibilities.

There are 2 basic approaches you can take. The first is to utilize randr which will allow you to create one large virtual display in which you can position your monitors. The second is to utilize xinerama which will allow you to have 3 separate X screens which will function as one large display. I will cover both here and you can decide which is more appropriate for your needs.

RandR - Resize and Rotate

The issue with XRandR is 2 fold in multi-monitor setups. It creates a large virtual display and each monitor gets a piece of it. If the total resolution is too high (2048 X 2048) it might not work... Framebuffer issues and such. You should be able to increase the fb size in xorg.conf but it comes at a performance loss iirc. Also, if the monitors are of different resolutions, there could potentially be areas that are either black, or windows fall off of your displays.

You can dynamically enable your multi-monitor setup by using XRandR

xrandr --output foo --rightof bar

xrandr --output baz --rightof foo

There is also a gui frontend for this called arandr which is in the 12.04 repos. I haven't tested it, but some have found it useful.

Or you could enable it statically in /etc/X11/xorg.conf.d/10-monitor.conf:

Section "Monitor"

  Identifier     "Monitor0"

EndSection



Section "Monitor"

  Identifier     "Monitor1"

  Option         "RightOf" "Monitor0" 

EndSection



Section "Monitor"

  Identifier     "Monitor2"

  Option         "RightOf" "Monitor1" 

EndSection

This method implies an randr setup. Again, there are some notable drawbacks if using xrandr with monitors of different resolutions. Windows may be improperly placed, there may be black areas on one screen, etc..

To enable xinerama you need to modify /etc/X11/xorg.conf:

Section "ServerFlags"

  Option    "Xinerama" "true"

EndSection

It might be as simple as that as X is pretty good about automagically configuring things based on hardware detection. But, if simply turning the xinerama extension on does not provide the functionality you desire, you may opt to fully configure X to use xinerama. There are a few ways to configure X. You could have multiple files in etc/X11/xorg.conf.d, some for your monitors, some for the devices, etc. I will cover having a single xorg.conf file that contains all the settings using a modified version of my current setup.

Section "ServerLayout"

    Identifier     "TriHead"

    Screen      0  "Screen0" 0 0

    Screen      1  "Screen1" RightOf "Screen0"

    Screen      2  "Screen1" RightOf "Screen1"

    Option         "Xinerama" "1"                # Enable xinerama on the whole

EndSection



Section "Device"

    Identifier     "Device0"

    Driver         "nvidia"                      # Be sure to write the correct driver here

    VendorName     "NVIDIA Corporation"          # Should be "intel" 

    BoardName      "Nvidia GT 240"

    BusID          "PCI:1:0:0"                   # Use lscpi | grep VGA to get the BusID

    Screen          0                            # Note the Screen numbers

EndSection



Section "Device"

    Identifier     "Device1"                     # A device for each screen

    Driver         "nvidia"

    VendorName     "NVIDIA Corporation"

    BoardName      "Nvidia GT 240"

    BusID          "PCI:1:0:0"

    Screen          1

EndSection



Section "Device"

    Identifier     "Device2"

    Driver         "nvidia"

    VendorName     "NVIDIA Corporation"

    BoardName      "Nvidia 8600 GT"

    BusID          "PCI:1:0:0"

    Screen          2

EndSection



Section "Monitor"

    Identifier     "Monitor0"

    VendorName     "Unknown"

    ModelName      "Gateway HX2000"

    HorizSync       31.0 - 83.0

    VertRefresh     56.0 - 76.0

    Option         "DPMS"

EndSection



Section "Monitor"

    Identifier     "Monitor1"

    VendorName     "Unknown"

    ModelName      "DELL 1907FPV"

    HorizSync       30.0 - 81.0

    VertRefresh     56.0 - 76.0

    Option         "DPMS"

EndSection



Section "Monitor"

    Identifier     "Monitor2"

    VendorName     "Unknown"

    ModelName      "DELL 1907FPV"

    HorizSync       30.0 - 81.0

    VertRefresh     56.0 - 76.0

    Option         "DPMS"

EndSection



Section "Screen"

    Identifier     "Screen0"

    Device         "Device0"

    Monitor        "Monitor0"

    DefaultDepth    24

EndSection



Section "Screen"

    Identifier     "Screen1"

    Device         "Device1"

    Monitor        "Monitor1"

    DefaultDepth    24

EndSection



Section "Screen"

    Identifier     "Screen2"

    Device         "Device2"

    Monitor        "Monitor2"

    DefaultDepth    24

EndSection

Obviously you will need to modify this to suit your hardware, but the basic skeleton is there for you.

Do note that I have recently read that Intel might have issues with xinerama. You may find that it is just not a possibility. Please test the answer I have given and continue to explore additional possibilities as you may find additional information elsewhere.