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 eμμ or μμμ is performed using data from pp collisions at √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⁺μ⁺μ^-, 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