Unlike the SugarCube story format, the release cycle of the Harlowe story format is generally tied to the release cycle of the Twine 2.x application. As the Twine v2.3.x application is still in beta testing, the Harlowe v3.0.0 story format hasn't really been released to public yet.
However due to the association between the GitHub repository web-site and the jsDelivr CDN web-site an installable pre-released copy of Harlowe is avaiable here.
WARNING: As the copy of the Harlowe 3.x found on the jsDelivr site is unofficial it may change at anytime without notice, use at your own digression.