  We study a new type of three-dimensional topological superconductors that
exhibit Majorana zero modes (MZM) protected by a magnetic group symmetry, a
combined antiunitary symmetry composed of a mirror reflection and
time-reversal. This new symmetry enhances the noninteracting topological
classification of a superconducting vortex from $Z_2$ to $Z$, indicating that
multiple MZMs can coexist at the end of one magnetic vortex of unit flux.
Specially, we show that a vortex binding two MZMs can be realized on the
$(001)$-surface of a topological crystalline insulator SnTe with proximity
induced BCS Cooper pairing, or in bulk superconductor In$_x$Sn$_{1-x}$Te.
