Invenio-Migrator should be installed on both your Invenio legacy installation and your new Invenio 3 installation.
You install Invenio-Migrator on Invenio legacy installations using PyPI:
$ pip install invenio-migrator
You install Invenio-Migrator on Invenio 3 installations using PyPI and the
extra dependencies loader
:
$ pip install invenio-migrator[loader]