  We perform an analytic and numerical study of parametric resonance in a
three-neutrino framework for sub-GeV neutrinos which travel through a periodic
density profile. Commensurate with the initial level of approximation, we
develop a parametric resonance condition similar to the exact condition for
two-neutrino systems. For a castle wall density profile, the \nu_e to \nu_\mu
oscillation probability is enhanced significantly and bounded by cos^2
\theta_{23}. The CP phase \delta enters into the oscillation probability as a
phase shift. For several cases, we examine the interplay between the
characteristics of the castle wall profile and the CP phase and determine which
profiles maximize the separation between oscillations with \delta = 0, \pi/2,
\pi. We also consider neutrinos which travel along a chord through the earth,
passing from the mantle to core and back to mantle again. Significant
enhancement of the oscillation probability is seen even in the case in which
the neutrino energy is far from the MSW resonant energies. At 500 GeV, the
difference between oscillation probabilities with \delta=0 and \delta=\pi/2 is
maximized.
