How do I upgrade to xcode 6 beta 3 when I already have beta 2 installed?

I am currently running xcode 6 beta 2 but would like to upgrade to the new beta 3 release. Do I have to uninstall beta 2 completely before doing a fresh beta 3 install, or is there a way to upgrade xcode beta 2 to the latest version?

The beta builds are independent of each other. You can have them installed concurrently or not.

Just drag and drop the beta you no longer want to the trash, then mount the latest version and copy it to your applications folder (or wherever you install your applications if different).

Make sure to update your commandline tools from the dev portal when applicable also (FYI thus far, Xcode 6 B3 has no commandline tool updates).

If you want to be pedantic over the uninstall process, in addition to the Applications directory, Xcode also installs files in the following directories:

/Users/[USER]/Library/Caches
/Users/[USER]/Library/Caches
~/private/var/folders/60/[key]/C
/Users/[USER]/Library/Caches/com.apple.dt.Xcode.savedState
/Users/[USER]/Library/Preferences
/Users/[USER]/Library/Application Support

If you want to remove them, the individual files themselves are easy to identify as their names mention 'xcode' at some point.

EDIT The new commandline tools for Xcode 6 B3 are now available: https://developer.apple.com/downloads/index.action?name=Xcode