“Edit the Pipfile” is the right way to go if you want to change the Python version of an existing environment.
If you want to create a new environment using Python 3.6, you can run
pipenv install --python 3.6
rather than
pipenv install --three
and that should do the trick.
Just be certain to delete the old Pipfile(s) if you create the new environment or else the commands will fail.