/usr/lib/x86_64-linux-gnu/libt3widget.so is in libt3widget-dev 0.6.2-1build1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libt3widget.so.1.0.0
/usr/lib/x86_64-linux-gnu/libt3widget.so.1.0.0