We have set up a new forum for MRtrix3, which we hope will provide a much better experience for our users than the current Google+ community page. This new forum is powered by Discourse, and provides vastly improved features. You should be able to log in using either your Google or GitHub credentials. Please take a look, and ask any questions you may have on this forum.
Thanks especially to Dave Raffelt and Rami Tabbara for their efforts in putting this together.