I assume you have cloned this repo (and the one for cogent3) to your local machine.
More details are on the c3dev wiki.
$ cd path/to/repos/c3dev
$ conda env create -f c3dev_environment.yml
will install the command line scripts into the active conda environment.
Then
$ cd path/to/repos/cogent3
$ flit install --deps all -s --python `which python`
installs cogent3
and all optional dependencies.
$ cd path/to/repos/c3dev
$ pip install -e .
followed by
$ cd path/to/repos/cogent3
$ flit install --deps all -s --python `which python`