/usr/lib/x86_64-linux-gnu/libfftw3_threads.so.3 is in libfftw3-double3 3.3.4-2ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libfftw3_threads.so.3.4.4
/usr/lib/x86_64-linux-gnu/libfftw3_threads.so.3.4.4