code: def tau3_from_mass1_mass2(mass1, mass2, f_lower):
      r
      mtotal = mass1 + mass2
      eta = eta_from_mass1_mass2(mass1, mass2)
      return tau3_from_mtotal_eta(mtotal, eta, f_lower)
docs: r"""Returns :math:`\tau_3` from the component masses and given frequency.