  In the early universe, neutrinos are slightly coupled when electron-positron
pairs annihilate transferring their entropy to photons. This process originates
non-thermal distortions on the neutrino spectra which depend on neutrino
flavour, larger for nu_e than for nu_mu or nu_tau. We study the effect of
three-neutrino flavour oscillations on the process of neutrino decoupling by
solving the momentum-dependent kinetic equations for the neutrino spectra. We
find that oscillations do not essentially modify the total change in the
neutrino energy density, giving N_eff=3.046 in terms of the effective number of
neutrinos, while the small effect over the production of primordial 4He is
increased by O(20%), up to 2.1 x 10^{-4}. These results are stable within the
presently favoured region of neutrino mixing parameters.
