How do I git pull a dev branch?
I'm not a Git expert, but as I understand it all of the releases of the dev branch listed on the project page are made directly to the 7.x-3.x branch.  Of course you're free to create your own branches locally for your own needs, but the master-like branch, which includes all of the releases pushed to drupal.org, is always going to be that 7.x-3.x branch.

If you want to switch to one of the tagged releases, use git tag to get a list of all the releases.  Then, to check out the 7.x-3.1 release (for example), use git co 7.x-3.1.

If you intend to edit the code for this release, you can then create a new branch (as Git explains) by using git checkout -b mycustombranch.