An expression for the characteristic rate of three-phonon processes in superfluid He4, which is valid in the entire range of phonon energies where three-phonon processes are allowed is derived proceeding from the hydrodynamic Landau Hamiltonian. Possible limiting cases are analyzed and compared with the results of previous investigations. It is found that three-phonon processes completely govern the initial relaxation of a phonon pulse injected into He II by a heated solid. As a result, the equilibrium form of phonon distribution is established in the anomalous region of phonon dispersion over a time interval of the order of 10−10 s.