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 root s = 13 TeV, recorded by the CMS detector at the LHC and corresponding to an integrated luminosity of 35.9 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 -> bH(+) -> bW(+)A -> bW(+)mu(+)mu(-), of 1.9 x 10(-6) to 8.6 x 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.