  Cygnus X-3 (Cyg X-3) is a well-known microquasar with relativistic jets. Cyg
X-3 is especially famous for its giant radio outbursts, which have been
observed once every few years since their first discovery. Each giant outburst
presumably consists of a series of short-duration flares. The physical
parameters of the flares in the giant outbursts are difficult to derive because
the successive flares overlap. Here, we report isolated flares in the quiescent
phase of Cyg X-3, as observed at 23, 43, and 86 GHz with the 45-m radio
telescope at Nobeyama Radio Observatory. The observed flares have small
amplitude (0.5--2 Jy) and short duration (1--2 h). The millimeter fluxes
rapidly increase and then exponentially decay. The lifetime of the decay is
shorter at higher frequency. The radio spectrum of Cyg X-3 during the flares is
flat or inverted around the peak flux density. After that, the spectrum
gradually becomes steeper. The observed characteristics are consistent with
those of adiabatic expanding plasma. The brightness temperature of the plasma
at the peak is estimated to be $T_B\gtrsim 1 \times 10^{11}$ K. The magnetic
field in the plasma is calculated to be $0.2 \lesssim H \lesssim 30$ G.
