You must have inadvertently upgraded your system pip (probably through something like
sudo pip install pip --upgrade)
pip 10.x adjusts where its internals are situated. The
pip3 command you’re seeing is one provided by your package maintainer (presumably debian based here?) and is not a file managed by pip.
You can read more about this on pip’s issue tracker
You’ll probably want to not upgrade your system pip and instead use a virtualenv.
To recover the
pip3 binary you’ll need to
sudo python3 -m pip uninstall pip && sudo apt install python3-pip --reinstall.
If you want to continue in “unsupported territory” (upgrading a system package outside of the system package manager), you can probably get away with
python3 -m pip ... instead of