A generalized treatment of the three‐wave decay of oblique whistler/electron cyclotron waves into other whistler/electron cyclotron waves is presented which clarifies the three‐dimensional aspects of the process. The generalized expression for the growth rate is given and several special cases are considered, establishing the existence of a strong instability. It is shown that there is no instability when the three wavenumber axes are coplanar with the background magnetic field in the plane except for the special case when one of the waves propagates along the background magnetic field.