/usr/lib/x86_64-linux-gnu/libpng.so.3 is in libpng3 1.2.54-1ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /lib/x86_64-linux-gnu/libpng12.so.0
/lib/x86_64-linux-gnu/libpng12.so.0