  We test the accuracy of ALMA flux density calibration by comparing ALMA flux
density measurements of extragalactic sources to measurements made by the
Planck mission; Planck is absolutely calibrated to sub-percent precision using
the dipole signal induced by the satellite's orbit around the solar system
barycenter. Planck observations ended before ALMA began systematic
observations, however, and many of the sources are variable, so we employ
measurements by the Atacama Cosmology Telescope (ACT) to bridge the two epochs.
We compare ACT observations at 93 and $\sim$145 GHz to Planck measurements at
100 and 143 GHz and to ALMA measurements made at 91.5 and 103.5 GHz in Band 3.
For both comparisons, flux density measurements were corrected to account for
the small differences in frequency using the best available spectral index for
each source. We find the ALMA flux density scale (based on observations of
Uranus) is consistent with Planck. All methods used to make the comparison are
consistent with ALMA flux densities in Band 3 averaging 0.99 times those
measured by Planck. One specific test gives ALMA/Planck = $0.996 \pm 0.024.$ We
also test the absolute calibration of both ACT at 93 and $\sim$145 GHz and the
South Pole Telescope (SPT) at 97.43, 152.9 and 215.8 GHz, again with reference
to Planck measurements at 100, 143 and 217 GHz, as well as the internal
consistency of measurements of compact sources made by all three instruments.
