  Neutrino oscillations are now a well-stablished and deeply studied phenomena.
Their mixing parameters, except for the CP phase, are measured with good
accuracy. The three-neutrino oscillation picture in matter is currently of
great interest due to the different long-baseline neutrino experiments that are
already running or under construction. In this work, we reanalyze the exact
expression for the neutrino probabilities (in a constant density medium) and
introduce an approximate formula. Our results are shown in a formulation that
is independent of the parametrization and could be useful for unitary tests of
the leptonic mixing matrix. We illustrate how the approximation, besides being
simple, can reproduce the neutrino probabilities with good accuracy.
