  Shr\"odinger equation for two-step spontaneous cascade transition in a
three-level quantum system is solved by means of Markovian approximation for
non-Markovian integro-differential evolution equations for amplitudes of
states. It is shown that both decay constant and radiation shift of initial
level are affected by instability of intermediate level of the cascade. These
phenomena are interpreted as the different manifestations of quantum Zeno-like
effect. The spectra of particles emitted during the cascade transition are
calculated in the general case and, in particular, for an unusual situation
when the initial state is lower than the intermediate one. It is shown that the
spectra of particles do not have a peak-like shape in the latter case.
