[en] The paper deals with the study on three-wave interactions in electron fluxes. Cases, when a fast cyclotron wave and a slow wave of a space charge or a fast wave of a space charge and a slow cyclotron wave interact correspondingly due to parametric coupling in the field of an electromagnetic pumping wave in a beam, are considered. The frequency band of the instability occurring is shown to be restricted from the side of high frequencies only by the limits of applicability of initial equations. Increments of instabilities are found, the efficiency of wave transformation at the linear stage of interaction is estimated