  Quantum fluctuations of a quantum Rabi triangle are studied using an
analytical approach beyond the mean-field theory. By applying an artificial
magnetic field among three cavities, time-reversal symmetry breaking is
manifested through a directional transfer dynamics of photons. In contrast to
previous studies, we focus on the scaling exponents of the fluctuations of the
local photon number and the position variance near the critical point. By
accurate calculation using Bogoliubov transformation we show that two scaling
laws emerge respectively for the frustrated cavity and the remaining cavities,
which are associated with the geometric frustrations. Especially, for the
frustrated cavity, the scaling exponent in the chiral superradiant phase is
different from that in the frustrated antiferromagnetic superradiant phase
without an artificial magnetic field. The unusual scaling exponents predict
distinct universality classes from the single-cavity Rabi universality. We
suggest that the accurate critical exponents in few-body system is useful for
identifying exotic quantum phase transition in light-matter coupling system.
