/usr/lib/libfftw3_threads.so.3 is in libfftw3-3 3.3-1ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/libfftw3_threads.so.3.3.0
/usr/lib/libfftw3_threads.so.3.3.0