The OPERA experiment is designed to search for nu(mu) -> nu(tau) oscillations in appearance mode, i.e., through the direct observation of the tau lepton in nu(tau)- charged current interactions. The experiment has taken data for five years, since 2008, with the CERN Neutrino to Gran Sasso beam. Previously, two nu(tau) candidates with a t decaying into hadrons were observed in a subsample of data of the 2008-2011 runs. Here we report the observation of a third nu(tau) candidate in the tau(-) -> mu(-) decay channel coming from the analysis of a subsample of the 2012 run. Taking into account the estimated background, the absence of nu(mu) -> nu(tau) oscillations is excluded at the 3.4 sigma level.