  On the basis of GWTC-3, we discuss the detection prospect of extra-Galactic
binary black holes (BBHs) by space gravitational-wave interferometers. In
particular, targeting BBHs with component masses around 5-100$M_\odot$, we
directly incorporate the chirp mass distribution of the 62 BBHs detected at
high significance. We find that, due to the reduction of both the comoving
merger rate and a weighted average of chirp masses, the expected detection
numbers are generally much smaller than the results obtained by the same
authors immediately after the report of GW150914. For LISA, the total BBH
detections are estimated to be $N_{\rm tot}\sim 2 (T/4{\rm yr})^{3/2}(\rho_{\rm
thr}/10)^{-3}$, dominated by nearly monochromatic BBHs ($\rho_{\rm thr}$: the
detection threshold, $T$: the observational period). TianQin will have a total
detection number $N_{\rm tot}$ similar to LISA. Meanwhile, TianQin has
potential to find $N_{\rm mer}\sim0.6 (T/4{\rm yr})^{7/4}(\rho_{\rm
thr}/10)^{-3}$ BBHs that merge in the observational period. This number for
merging BBHs is 4-5 times larger than that of LISA, because of the difference
between the optimal bands. We also investigate prospects for joint operations
of multiple detectors, finding that concurrent observations will be more
advantageous than sequential ones.
