This is how I setup my installation - a 2 step process.
1. Find the .dmg files which is downloaded by XCode 4.2 and download it seperately.
2. Spoof your hosts file of the machine to point the files you have downloaded already.
Direct download link for 4.3
Direct download link for 3.x
By inspecting the console.app I was able to figure out the URL of .dmg xcode trying to download.
Eg. The path for 4x simulator is
which resolves into
#xcode4.2 legacy simulators dmg proxy 127.0.0.1 adcdownload.apple.com