The v3 BEP has been drafted and is available at:
https://tixati.com/specs/bittorrent/bep-v3.html
or in .rst (ReStructuredText) format:
https://tixati.com/specs/bittorrent/bep-v3.rst
I will submit a pull request soon via github, but there are already several others in the queue, some from a long time ago. It appears the bittorrent.org repo has been unmaintained for quite some time.
The original spec at
https://tixati.com/specs/bittorrent/v3 is the most detailed documentation, although it is a very simple protocol addition. It is not difficult at all for any developer to implement in an existing client.
I will also draft a BEP for v3.1 in the coming weeks.