Using the Belle II data from the early Phase III data taking, we reconstructed the tau
leptons using the 3-prong τ decays. This decay mode is used for the tau-lepton mass
measurement using the pseudomass technique developed by the ARGUS experiment. Though
this measurement is expected to be limited by statistics and imperfect knowledge of
the detector performance, we foresee that Belle II will provide the best tau mass measurement using the pseudomass technique once a larger data set with fully understood and operational detector
components is available.