  We find a superradiant quantum phase transition in the model of triangular
molecular magnets coupled to the electric component of a microwave cavity
field. The transition occurs when the coupling strength exceeds a critical
value which, in sharp contrast to the standard two-level emitters, can be tuned
by an external magnetic field. In addition to emitted radiation, the molecules
develop an in-plane electric dipole moment at the transition. We estimate that
the transition can be detected in state of the art microwave strip-line
cavities containing $10^{15}$ molecules.
