  The interplay between non-trivial topology and strong electron interaction
can generate a variety of exotic quantum matter. Here we theoretically propose
that monolayer transition metal trihalides MoF$_3$ and W$X_3$ ($X$= Cl, Br, I)
have isolated nearly flat band near the Fermi level with higher Chern number
$\mathcal{C}=+3$ and $\mathcal{C}=-2$, respectively. The nontrivial topology of
these flat Chern bands originates from the effective $sd^2$ hybridization of
transition metal atom, which transform the apparent atomic $d$ orbitals on a
hexagonal lattice into $(s, p_+, p_-)$ orbitals on a triangular lattice.
Interestingly, the quantum geometry of flat Chern bands in these materials are
comparable with those in moir\'e systems exhibiting fractional Chern insulator
state. The Hofstadter butterfly of such flat Chern bands are further studied.
These natural materials, if realized experimentally, could offer new platforms
to explore correlated phenomena driven by flat Chern band with higher Chern
number.
