  We study the three-body baryonic $B\to {\bf B\bar B'}M$ decays with $M$
representing the $\eta$ or $\eta'$ meson. Particularly, we predict that ${\cal
B}(B^-\to\Lambda\bar p\eta,\Lambda\bar p\eta')=(5.3\pm 1.4,3.3\pm 0.7)\times
10^{-6}$ or $(4.0\pm 0.7,4.6\pm 1.1)\times 10^{-6}$, where the errors arise
from the non-factorizable effects as well as the uncertainties in the $0\to
{\bf B\bar B'}$ and $B\to{\bf B\bar B'}$ transition form factors, while the two
different results are due to overall relative signs between the form factors,
causing the constructive and destructive interference effects. For the
corresponding baryonic $\bar B_s^0$ decays, we find that ${\cal B}(\bar
B^0_s\to \Lambda\bar \Lambda \eta,\Lambda\bar \Lambda \eta')=(1.2\pm 0.3,2.6\pm
0.8)\times 10^{-6}$ or $(2.1\pm 0.6,1.5\pm 0.4)\times 10^{-6}$ with the errors
similar to those above. The decays in question are accessible to the
experiments at BELLE and LHCb.
