  We analyse tau -> (3 pions) nu_tau decays within the framework of the
resonance chiral theory of QCD. We have worked out the relevant Lagrangian that
describes the axial-vector current hadronization contributing to these
processes, and the new coupling constants that arise have been constrained by
imposing the asymptotic behaviour of the corresponding spectral function within
QCD. Hence we compare the theoretical framework with the experimental data,
obtaining a good quality fit from the ALEPH spectral function and branching
ratio. We also get values for the mass and on-shell width of the a_1(1260)
resonance, and provide the tau -> (3 pions) nu_tau structure functions that
have been measured by OPAL and CLEO-II finding an excellent agreement.
