  We investigate neutrino oscillations in constant matter within the context of
the standard three neutrino scenario. We derive an exact and simple formula for
the oscillation probability applicable to all channels. In the standard
parametrization, the probability for $\nu_e$ $\to$ $\nu_{\mu}$ transition can
be written in the form $P(\nu_e \to
\nu_{\mu})=A_{e\mu}\cos\delta+B_{e\mu}\sin\delta+C_{e\mu}$ without any
approximation using CP phase $\delta$. For $\nu_{\mu}$ $\to$ $\nu_{\tau}$
transition, the linear term of $\cos 2\delta$ is added and the probability can
be written in the form $P(\nu_{\mu} \to
\nu_{\tau})=A_{\mu\tau}\cos\delta+B_{\mu\tau}
\sin\delta+C_{\mu\tau}+D_{\mu\tau}\cos 2\delta$. We give the CP dependences of
the probability for other channels. We show that the probability for each
channel in matter has the same form with respect to $\delta$ as in vacuum. It
means that matter effects just modify the coefficients $A$, $B$, $C$ and $D$.
We also give the exact expression of the coefficients for each channel.
Furthermore, we show that our results with respect to CP dependences are
reproduced from the effective mixing angles and the effective CP phase
calculated by Zaglauer and Schwarzer. Through the calculation, a new identity
is obtained by dividing the Naumov-Harrison-Scott identity by the Toshev
identity.
