  Results are reported from a search for the lepton flavor violating decay
$\tau$ $\to$ 3$\mu$ in proton-proton collisions at $\sqrt{s} =$ 13 TeV. The
data sample corresponds to an integrated luminosity of 33.2 fb$^{-1}$ recorded
by the CMS experiment at the LHC in 2016. The search exploits $\tau$ leptons
produced in both W boson and heavy-flavor hadron decays. No significant excess
above the expected background is observed. An upper limit on the branching
fraction $\mathcal{B}(\tau$ $\to$ 3$\mu)$ of 8.0 $\times$ 10$^{-8}$ at 90%
confidence level is obtained, with an expected upper limit of 6.9 $\times$
10$^{-8}$.
