The current stable version is 3.α (transcribed 3.a because many systems still can’t handle UTF-8 in version numbers).
Use git to follow the latest changes:
$ git clone git://code.stapelberg.de/i3
If you prefer to download a tarball, or if you cannot use git for whatever reason, you may download the current master branch from http://code.stapelberg.de/git/i3/snapshot/i3-master.tar.bz2