Version "3" of Xdmf is the master branch of: git://xdmf.org/Xdmf.git
The initial plans for changes that went into version 3 are summarized on: 
The overall goals included better performance and use of modern and more maintainable C++ programming idioms. This version adds a dependence on boost.
Additionally, this version added support for Graph types and removed support for the SQL back end.