  We report on the serendipitous discovery of three transient mm-wave sources
using data from the Atacama Cosmology Telescope. The first, detected at RA =
273.8138, dec = -49.4628 at ${\sim}50\sigma$ total, brightened from less than 5
mJy to at least 1100 mJy at 150 GHz with an unknown rise time shorter than
thirteen days, during which the increase from 250 mJy to 1100 mJy took only 8
minutes. Maximum flux was observed on 2019-11-8. The source's spectral index in
flux between 90 and 150 GHz was positive, $\alpha = 1.5\pm0.2$. The second,
detected at RA = 105.1584, dec = -11.2434 at ${\sim}20\sigma$ total, brightened
from less than 20 mJy to at least 300 mJy at 150 GHz with an unknown rise time
shorter than eight days. Maximum flux was observed on 2019-12-15. Its spectral
index was also positive, $\alpha = 1.8\pm0.2$. The third, detected at RA =
301.9952, dec = 16.1652 at ${\sim}40\sigma$ total, brightened from less than 8
mJy to at least 300 mJy at 150 GHz over a day or less but decayed over a few
days. Maximum flux was observed on 2018-9-11. Its spectrum was approximately
flat, with a spectral index of $\alpha = -0.2\pm0.1$. None of the sources were
polarized to the limits of these measurements. The two rising-spectrum sources
are coincident in position with M and K stars, while the third is coincident
with a G star.
