The interaction in a plasma between high-frequency and low-frequency waves with given phases is considered in a three-wave approximation. The number of interacting waves is arbitrary. Linear damping of the waves is taken into account. Approximate solutions are found of the equations for the amplitudes. A criterion is established for the decay instability of a strong high-frequency wave. The author finds the equilibrium distribution of energy among the waves and the criterion for its establishment. The results are illustrated by examples of the interaction between transverse electromagnetic and Langmuir waves in a homogeneous equilibrium plasma and of the interaction of space charge waves with ion-acoustic waves in a conducting cylinder situated in a longitudinal magnetic field. Comparisons are made with experiment.