  All existing solar and atmospheric neutrino oscillation data are described by
the standard two-neutrino formulas with neutrino mass squared differences that
satisfy the hierarchy relation $\Delta m^{2}_{sol} \ll \Delta m^{2}_{atm}$. We
discuss this phenomenon from the point of view of three-neutrino mixing.
Possibilities to see effects of three-neutrino mixing and, in particular,
effects of CP-violation in the lepton sector depend on the values of
$|U_{e3}|^2 $ and $\Delta m^{2}_{sol}$. We present an upper bound of
$|U_{e3}|^2 $, which was obtained from a three-neutrino analysis of CHOOZ data
in the case if the solar neutrino oscillation parameters lie in the LMA allowed
region.
