The maturity level of MathML 3 is now "Candidate Recommendation". This means that we are invited to implement the new features. Of course, this bug depends on bug 525772 (complete support for MathML 2). Also, note that support for Content MathML (bug 276028) or MathML DOM (bug 143842) is excluded from this plan.
A list of changes is given in the appendix: http://www.w3.org/TR/MathML3/appendixf.html