Where is the source code of version 3.1 and 3.x?

[yu huang]

hi,
i'm trying to make some contribution. But i find that there are many issues
about the version 3.1 and 3.x,and can't find the source code of 3.1 and 3.x
in svn. So where is the source code of version 3.1 and 3.x?