Or you could just use the the Postgres APT repo, as example:https://apt.postgresql.org/pub/repos/apt/dists/stretch-pgdg/9.3/ <https://apt.postgresql.org/pub/repos/apt/dists/stretch-pgdg/9.3/>see also https://wiki.postgresql.org/wiki/Apt/FAQ#Where_are_older_versions_of_the_packages.3F <https://wiki.postgresql.org/wiki/Apt/FAQ#Where_are_older_versions_of_the_packages.3F>
I discovered that Debian allows to install 9.3 side-by side with 12 apt-get install postgresql-9.3 Andrus.