A search for the lepton flavor violating tau -> 3 mu decay is performed using proton-proton collision events at a center-of-mass energy of 13 TeV collected by the CMS experiment at the LHC in 2017-2018, corresponding to an integrated luminosity of 97.7 fb(-1). Tau leptons produced in both heavy-flavor hadron and W boson decays are exploited in the analysis. No evidence for the decay is observed. The results of this search are combined with an earlier null result based on data collected in 2016 to obtain a total integrated luminosity of 131 fb(-1). The observed (expected) upper limits on the branching fraction.( tau -> 3 mu) at confidence levels of 90 and 95% are 2.9 x 10(-8) (2.4 x 10(-8)) and 3.6 x 10(-8) (3.0 x 10(-8)), respectively.