  We have performed a time-resolved and phase-sensitive investigation of
three-magnon scattering of ferromagnetic resonance (FMR) over several orders of
magnitude in excitation power. We observe a regime that hosts transient
oscillations of the FMR magnon population, despite higher-order magnon
interactions at large powers. Also at high powers, the scattering generates
$180^\circ$ phase shifts of the FMR magnons. These phase shifts correspond to
reversals in the three-magnon scattering direction, between splitting and
confluence. These scattering reversals are most directly observed after
removing the microwave excitation, generating coherent oscillations of the FMR
magnon population much larger than its steady-state value during the
excitation. Our model is in strong agreement with these findings. These
findings reveal the transient behavior of this three-magnon scattering process,
and the nontrivial interplay between three-magnon scattering and the magnons'
phases.
