The Seam 3 source code was (finally) migrated to Git version control system on Oct 16, 2010. You can find all the Seam 3 modules, examples and auxiliary projects under the Seam organization at github.com.
If you just want to build the source code, use the checkout script to clone all the repositories (clone is the git equivalent to SVN checkout). If you want to submit changes, follow the forking process as described in the github help pages. More information about building Seam 3 can be found on the Seam 3 build pages.