the SNO solar neutrino experiment [@SNO2; @SNO3] (see also [@SNO1]) provided further strong evidences for oscillations or transitions of the solar $\nu_e$ into active neutrinos $\nu_{\mu (\tau)}$ (and/or antineutrinos $\bar{\nu}_{\mu (\tau)}$). These evidences become even stronger when the SNO data are combined with the data obtained in the other solar neutrino experiments, Homestake, Kamiokande, SAGE, GALLEX/GNO and Super-Kamiokande [@SKsol; @Cl98]. As the two-neutrino oscillation analyzes of the solar neutrino data show (see, e.g., [@SNO2]), the latter favor the large mixing angle (LMA) MSW $\nu_e \rightarrow \nu_{\mu (\tau)}$ transition solution with $\Delta m^2_{\odot} \sim 5\times 10^{-5}~{\rm eV^2}$ and $\tan^2\theta_{\odot} \sim 0.33$, $\tan^2\theta_{\odot} < 1$, where $\Delta m^2_{\odot}$ and $\theta_{\odot}$ are the neutrino mass squared difference and mixing angle which control the solar neutrino transitions. Strong evidences for oscillations of atmospheric neutrinos have been obtained in the Super-Kamiokande experiment [@SKatm00]. The atmospheric neutrino data, as is well known, is best described in terms of dominant $\nu_{\mu} \rightarrow \nu_{\tau}$ ($\bar{\nu}_{\mu} \rightarrow \bar{\nu}_{\tau}$) oscillations with $|\Delta m^2_{\mathrm{atm}}| \sim (2.5 - 3.0)\times 10^{-3}~{\rm eV^2}$.

The explanation of the solar and atmospheric neutrino data in terms of neutrino oscillations requires the existence of 3-neutrino mixing in the weak charged lepton current (see, e.g., [@BGG99; @P99]): $$\nu_{l 