A search for a light charged Higgs boson (({H}^{+})) decaying to a (W) boson and a (CP)-odd Higgs boson ((A)) in final states with (e\mu \mu ) or (\mu \mu \mu ) is performed using data from (pp) collisions at (\sqrt{s}=13\text{}\text{}\mathrm{TeV}), recorded by the CMS detector at the LHC and corresponding to an integrated luminosity of (35.9\text{}\text{}{\mathrm{fb}}^{-1}). In this search, it is assumed that the ({H}^{+}) boson is produced in decays of top quarks, and the (A) boson decays to two oppositely charged muons. The presence of signals for ({H}^{+}) boson masses between 100 and 160 GeV and (A) boson masses between 15 and 75 GeV is investigated. No evidence for the production of the ({H}^{+}) boson is found. Upper limits at 95% confidence level are obtained on the combined branching fraction for the decay chain, (t\to b{H}^{+}\to b{W}^{+}A\to b{W}^{+}{\mu }^{+}{\mu }^{-}), of (1.9×{10}^{-6}) to (8.6×{10}^{-6}), depending on the masses of the ({H}^{+}) and (A) bosons. These are the first limits for these decay modes of the ({H}^{+}) and (A) bosons.