The short answer at the moment is - you can't. They haven't quite finished the port to python3 yet.
There is alleged compatibility in the development branch but the recommended manual setup didn't work for me (in virtualenv) - it fails in a different way.
https://github.com/clips/pattern/tree/development
The porting issue thread is here, spanning 2013 to yesterday:
https://github.com/clips/pattern/issues/62
The official A contributing port repo is here, but it's not finished yet (the readme says there is no Python3 support).
https://github.com/pattern3/pattern
So you could try pip install pattern3
which does install it - but it has a different package name, so you would have to modify any references to it. For me this is "impossible" as it's required by other 3rd party packages like GenSim.
UPDATE
I did get it working in the end in Python3 with Gensim, by manually installing it from the development branch as suggested and fixing up a few problems both during install and execution. (I removed the mysql-client dependency as the installer isn't working on Mac. I manually downloaded the certs for the NTLK wordnet corpus, to fix an SSL error in the setup. I also fixed a couple of scripts which were erroring, e.g. an empty 'try' clause in tree.py). It has a huge set of dependencies!
After reading more on the port activity, it seems they're almost finished and should release in a few months (perhaps early 2018). Furthermore the pattern3 repository is more of a "friend" than the official Python3 fork. They have already pulled the changes from this contributor into the main repo and they are preparing to get it released.
Therefore it should become available on pip
in the main pattern
package (not the pattern3 one which I presume will be removed), and there should be no package name-change problems.