As a general rule of thumb, it’s best to stick with the latest release of Xcode. That said, there are times when running an older release can be a good thing.
You can download older versions of Xcode by logging into Apple Developer Connection – Downloads and ADC Program Assests (http://connect.apple.com) – You will need to be a member of the developer program to login.
Once on the site, from the Downloads section, choose Developer Tools – you will be shown with a list of all Xcode versions, back to 1.0 released in October of 2004.
Install Multiple Versions of Xcode
To keep from overwriting an existing installation, you can specify the path for Xcode during the install process. For example, the default location is /Developer, to install an older version you could instruct Xcode to install into a simliar name such as /Developer-3.1
To change the install path, click on the Location header in the install dialog:
Choose a new path for the Essentials Package: