  In this work, we study the topological properties and magnon Hall effect of a
three-dimensional ferromagnet in the ABC stacking honeycomb lattice, motivated
by the recent inelastic neutron scattering study of CrI$_3$. We show that the
magnon band structure and Chern numbers of the magnon branches are
significantly affected by the interlayer coupling $J_c$, which moreover has a
qualitatively different effect in the ABC stacking compared to the AA stacking
adopted by other authors. The nontrivial Chern number of the lowest magnon band
is stabilized by the next-nearest-neighbour Dzyaloshinsky-Moriya interaction in
each honeycomb layer, resulting in the hopping term similar to that in the
electronic Haldane model for graphene. However, we also find several gapless
Weyl points, separating the non-equivalent Chern insulating phases, tuned by
the ratio of the interlayer coupling $J_c$ and the third-neighbour Heisenberg
interaction $J_3$. We further show that the topological character of magnon
bands results in non-zero thermal Hall conductivity, whose sign and magnitude
depend on $J_c$ and the intra-layer couplings. Since the interlayer coupling
strength $J_c$ can be easily tuned by applying pressure to the quasi-2D
material such as CrI$_3$, this provides a potential route to tuning the magnon
thermal Hall effect in an experiment.
