Nitrokey App 2 is required for Nitrokey 3 devices. Download it here.
Nitrokey App 1 is required for Nitrokey Pro and Nitrokey Storage only.
The latest Nitrokey App is version (Release notes, PGP signatures and older versions).
Released binaries are signed by PGP key:
868184069239FF65DE0BCD7DD9BAE35991DE5B22