  We analyse the tidal disruption probability of potential neutron star--black
hole (NSBH) merger gravitational wave (GW) events, including GW190426_152155,
GW190814, GW200105_162426 and GW200115_042309, detected during the third
observing run of the LIGO/Virgo Collaboration, and the detectability of
kilonova emission in connection with these events. The posterior distributions
of GW190814 and GW200105_162426 show that they must be plunging events and
hence no kilonova signal is expected from these events. With the stiffest NS
equation of state allowed by the constraint of GW170817 taken into account, the
probability that GW190426_152155 and GW200115_042309 can make tidal disruption
is $\sim24\%$ and $\sim3\%$, respectively. However, the predicted kilonova
brightness is too faint to be detected for present follow-up search campaigns,
which explains the lack of electromagnetic (EM) counterpart detection after
triggers of these GW events. Based on the best constrained population synthesis
simulation results, we find that disrupted events account for only
$\lesssim20\%$ of cosmological NSBH mergers since most of the primary BHs could
have low spins. The associated kilonovae for those disrupted events are still
difficult to be discovered by LSST after GW triggers in the future, because of
their low brightness and larger distances. For future GW-triggered
multi-messenger observations, potential short-duration gamma-ray bursts and
afterglows are more probable EM counterparts of NSBH GW events.
