  We analyse new signals of Dark Matter (DM) at the Large Hadron Collider (LHC)
in a 3-Higgs Doublet Model (3HDM) where only one doublet acquires a Vacuum
Expectation Value (VEV), preserving a parity $Z_2$. The other two doublets are
inert and do not develop a VEV, leading to a dark scalar sector controlled by
$Z_2$, with the lightest CP-even dark scalar $H_1$ being the DM candidate. This
leads to the loop induced decay of the next-to-lightest scalar, $H_2\to
H_1\bar{f}f$ ($f=u,d,c,s,b,e,\mu,\tau$), mediated by both dark CP-odd and
charged scalars. This is a smoking-gun signal of the 3HDM since it is not
allowed in the 2HDM with one inert doublet and is expected to be important when
$H_2$ and $H_1$ are close in mass. In practice, this signature can be observed
in the cascade decay of the SM-like Higgs boson, $h\to H_1H_2\to
H_1H_1\bar{f}f$ into two DM particles and di-leptons/di-jets, where $h$ is
produced from either gluon-gluon Fusion (ggF) or Vector Boson Fusion (VBF).
