  CONTEXT: The unparalleled characteristics of Gaia photometry make it an
excellent choice to study stellar variability. AIMS: To measure the phot.
dispersion in G+G_BP+G_RP of the 145 677 450 Gaia DR3 5-parameter sources with
G <= 17 mag and G_BP-G_RP with -1.0 to 8.0 mag. To use that unbiased sample to
analyze stellar variability in the Milky Way, LMC, and SMC. METHODS: We convert
from magnitude uncertainties to the observed phot. dispersions, calculate the
instrumental component as a function of apparent magnitude and color, and use
it to transform the observed dispersions into the astrophysical ones. We give
variability indices in the three bands for the whole sample. We use the
subsample of Rimoldini et al. that includes light curves and variability types
to calibrate our results and establish their limitations. RESULTS: We use
information from the MW, LMC, and SMC CAMDs to discuss variability across the
HRD. WDs and sdBs are more variable than MS or RC stars but their variability
decreases for the former with age. The MS region in the Gaia CAMD has an s_G
distribution peaks at low values (~1-2 mmag) and has a large tail dominated by
EBs, RR Lyr stars, and YSOs. RC stars are characterized by little variability,
with their s_G distribution peaking at 1 mmag or less. The stars in the PMS
region are highly variable, with a power law distribution in s_G with slope
2.75 and a cutoff for values lower than 7 mmag. The luminous red stars region
of the Gaia CAMD has the highest variability, with its extreme dominated by AGB
stars and with a power law in s_G with a slope of ~2.2 that extends from there
to a cutoff of 7 mmag. We show that our method can be used to search for LMC
Cepheids. We analyze four stellar clusters with O stars and detect a strong
difference in s_G between stars that are already in the MS and those that are
still in the PMS. [ABRIDGED BUT NOT TOO FAR]
