  Quantum Griffiths singularity (QGS) has been experimentally observed in a
range of two-dimensional (2D) superconducting systems. Although it is
theoretically suggested that the QGS also exists in three-dimensional (3D)
superconductors, there is almost no experimental support to the theoretical
prediction. In the present paper, we observe the occurrence of QGS in a series
of $\sim$80-nm-thick Mo$_{0.8}$Ti$_{0.2}$N$_x$ ($0.84 \lesssim x \lesssim
1.12$) superconducting films near the field-driven superconductor-metal
transition (SMT). These films have a NaCl structure and are 3D with respect to
the superconductivity. For each film, the low-temperature magnetoresistance
isotherms, measured at magnetic fields being perpendicular or parallel to the
film plane, do not cross at a single point but at a clear wide region. The
dynamical critical exponents $z\nu_{\perp}$ (for perpendicular field) and
$z\nu_{\parallel}$ (for parallel field) obtained by analyzing the related
magnetoresistance isotherms increase with decreasing temperature and tend to
diverge as $T\rightarrow 0$ K. In addition, the effective resistivity data for
the perpendicular and parallel field in the vicinity of the SMTs both obey an
activated scaling based on the random transverse-field Ising model. We also
fabricate a $\sim$80-nm-thick (Mo$_{0.8}$Ti$_{0.2}$)$_2$N$_{1.06}$
superconducting film with face-centered cubic structure at low nitrogen partial
pressure. It is found that the low-temperature magnetoresistance isotherms for
the perpendicular (parallel) field cross at a single point and the resistivity
data for the perpendicular (parallel) field in the vicinity of the
field-induced SMT obey the power-law scaling deduced from the dirty-boson
model. Our results provide unambigous experimental evidence for the existence
of QGS in 3D superconductors.
