We analyse new signals of Dark Matter (DM) at the LHC, in a 3-Higgs Doublet Model (3HDM) in which only one doublet acquires a Vacuum Expectation Value (VEV), preserving a Higgs parity Z_2. The other two doublets are inert and do not develop a VEV, leading to a dark scalar sector controlled by the Higgs parity and a Dark Matter (DM) candidate provided by the lightest CP-even dark scalar H1. This leads to a loop induced decay of the next-to-lightest scalar, H_2 \to H_1 \gamma^* \to H_1 l+l- mediated by loops involving both the dark CP-odd and charged scalars. This smoking gun decay, not permitted in the 2HDM with one inert doublet, is expected to be important when H2 and H1 are close in mass. We examine the significance of this channel in the light of the LHC Run 2.