  We present a theoretical study of a quantum regime of the resistive
(whirling) state of dc driven anisotropic single plaquette containing three
small Josephson junctions. The current-voltage characteristics of such a system
display resonant steps that are due to the resonant interaction between the
time dependent Josephson current and the excited electromagnetic oscillations
(EOs). The voltage positions of the resonances are determined by the quantum
interband transitions of EOs. We show that in the quantum regime as the system
is driven on the resonance, coherent Rabi oscillations between the quantum
levels of EOs occur. At variance with the classical regime the magnitude and
the width of resonances are determined by the frequency of Rabi oscillations
that in turn, depends in a peculiar manner on an externally applied magnetic
field and the parameters of the system.
