Symmetry-protected topological (SPT) phases of bosons have been widely studied in recent
years. Extensions of these studies into fermionic systems turn out to be not very
straightforward. In this talk, I will discuss some aspects of fermionic SPTs with unitary
symmetries in three dimensions, whose existence inevitably requires strong interaction. Both
SPT phases with internal and spatial symmetries (e.g., rotations) will be touched. Two results
will be emphasized: (1) After gauging the internal symmetries, there exist certain loop-like
gauge fluxes, which carry Majorana zero modes if linked with some other loops, but not if
unlinked. (2) For the recently proposed correspondence principle between crystalline
topological phases and those with internal symmetries to be precisely hold, we show that
there is a subtlety involving the central extension structure of the symmetry group by the Z2
fermion parity.