Default installation of dist Ubuntu 16.04 has /usr/lib/python3 installed but nothing inside of /usr/bin nor /usr/local/bin.
Why would they include a lib folder with python3 and python2 without installing python? This is the Docker Ubuntu image if that has any effect.
You can list all of the installed packages which use /usr/lib/python3 with this command:
dpkg -S /usr/lib/python3
If you run that command on your system, perhaps its output will provide insight into why that directory exists there. One one of my Ubuntu systems /usr/lib/python3 contains only dist-packages, with files beneath dist-packages owned by a variety of packages. It's possible some packages on your system provide support for python3 without requiring python3 itself to be installed.
