Three neutrino flavor oscillations and the atmospheric tau neutrinos

Downward going atmospheric tau neutrino flux is estimated in the presence of three neutrino flavor oscillations for 1 GeV < E < 10^3 GeV. The relative differences between the three and purely two neutrino flavor oscillations are elaborated. As an implication, the downward going atmospheric tau neutrino flux is compared with the galactic plane tau neutrino flux that is also estimated in the presence of three neutrino flavor oscillations. It is pointed out that the galactic plane tau neutrino flux dominates over the downward going atmospheric tau neutrino flux until E \sim 10 GeV.