It is basically the same trick as with Lion, except that it renders your system unbootable. Oops. Be prepared to boot into single-user mode and remove the incompatible kernel extensions.

Note that Interface Builder plugins from iOS platform won’t load, so you can’t use iOS SDK with Xcode 3 anymore.

1. Make sure you know how to boot into single-user mode (hit Cmd+S on boot).

2. Save a repair script somewhere (e.g. /Users/my-user-name/repair-script):

    fsck -fy
    mount -uw /
    rm -rf /System/Library/Extensions/AppleProfileFamily.kext
    rm -rf /System/Library/Extensions/CHUD*.kext
   

3. If Xcode 3 is on a DMG, mount it.

4. Open Terminal and cd to whereever the installation package is.

    $ cd "/Volumes/Xcode and iOS SDK"
    $ ls -1
    About Xcode and iOS SDK.pdf
    Packages
    Xcode and iOS SDK.mpkg
   

5. Run the installation package like this:

    $ COMMAND_LINE_INSTALL=1 open "Xcode and iOS SDK.mpkg"
   

6. Uncheck iOS SDK option, or the Interface Builder will not be able to start.

7. You get kernel panic and your system won’t boot anymore. Hooray. Turn the power off.

8. Turn the power back on and hit Cmd+S once gray screen appears, then run the repair script:

    # . /Users/my-user-name/repair-script
    # exit
   

Done.