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). However, this signal competes with the tree-level channel $q\bar{q}\to H_1H_1Z^∗\to H_1H_1\bar{f}f$. We devise some benchmarks, compliant with collider, DM and cosmological data, for which the interplay between these modes is discussed. In particular, we show that the resulting detector signature, with missing energy and invariant mass of $\bar{f}f$ much smaller than $m_Z$, can potentially be extracted already during Run 2 and 3. For example, the $H_2\to H_1\gamma^{∗}$ and $\gamma^{∗}\to e^+e^−$ case will give a spectacular QED mono-shower signal.