  High-angular-resolution cosmic microwave background experiments provide a
unique opportunity to conduct a survey of time-variable sources at millimeter
wavelengths, a population which has primarily been understood through follow-up
measurements of detections in other bands. Here we report the first results of
an astronomical transient survey with the South Pole Telescope (SPT) using the
SPT-3G camera to observe 1500 square degrees of the southern sky. The
observations took place from March to November 2020 in three bands centered at
95, 150, and 220 GHz. This survey yielded the detection of fifteen transient
events from sources not previously detected by the SPT. The majority are
associated with variable stars of different types, expanding the number of such
detected flares by more than a factor of two. The stellar flares are
unpolarized and bright, in some cases exceeding 1 Jy, and have durations from a
few minutes to several hours. Another population of detected events last for
2--3 weeks and appear to be extragalactic in origin. Though data availability
at other wavelengths is limited, we find evidence for concurrent optical
activity for two of the stellar flares. Future data from SPT-3G and forthcoming
instruments will provide real-time detection of millimeter-wave transients on
timescales of minutes to months.
