To use FFTW 3.x, load the module cray-fftw or cray-fftw/<version>
In FFTW 3.x, the single- and double-precision routine names are unique, and therefore both libraries automatically appear on the link line when the fftw module is loaded, so that a user's program can call single- or double-precision routines. The single- and double-precision libraries are libfftw3f.a and libfftw3.a, respectively.
Additional details are available in the intro_fftw3 man pages on Cori and Edison.