  We have performed monitoring observations of the 3-mm flux density toward the
Galactic Center compact radio source Sgr A* with the Australia Telescope
Compact Array since 2005 October. Careful calibrations of both
elevation-dependent and time-dependent gains have enabled us to establish the
variability behavior of Sgr A*. Sgr A* appeared to undergo a high and stable
state in 2006 June session, and a low and variable state in 2006 August
session. We report the results, with emphasis on two detected intra-day
variation events during its low states. One is on 2006 August 12 when Sgr A*
exhibited a 33% fractional variation in about 2.5 hr. The other is on 2006
August 13 when two peaks separated by about 4 hr, with a maximum variation of
21% within 2 hr, were seen. The observed short timescale variations are
discussed in light of two possible scenarios, i.e., the expanding plasmon model
and the sub-Keplerian orbiting hot spot model. The fitting results indicate
that for the adiabatically expanding plasmon model, the synchrotron cooling can
not be ignored, and a minimum mass-loss rate of 9.7*10^{-10}M_sun /yr is
obtained based on parameters derived for this modified expanding plasmon model.
Simultaneous multi-wavelength observation is crucial to our understanding the
physical origin of rapid radio variability in Sgr A*.
