  We present an incremental version (4FGL-DR3, for Data Release 3) of the
fourth Fermi-LAT catalog of gamma-ray sources. Based on the first twelve years
of science data in the energy range from 50 MeV to 1 TeV, it contains 6658
sources. The analysis improves on that used for the 4FGL catalog over eight
years of data: more sources are fit with curved spectra, we introduce a more
robust spectral parameterization for pulsars, and we extend the spectral points
to 1 TeV. The spectral parameters, spectral energy distributions, and
associations are updated for all sources. Light curves are rebuilt for all
sources with 1 yr intervals (not 2 month intervals). Among the 5064 original
4FGL sources, 16 were deleted, 112 are formally below the detection threshold
over 12 yr (but are kept in the list), while 74 are newly associated, 10 have
an improved association, and seven associations were withdrawn. Pulsars are
split explicitly between young and millisecond pulsars. Pulsars and binaries
newly detected in LAT sources, as well as more than 100 newly classified
blazars, are reported. We add three extended sources and 1607 new point
sources, mostly just above the detection threshold, among which eight are
considered identified, and 699 have a plausible counterpart at other
wavelengths. We discuss degree-scale residuals to the global sky model and
clusters of soft unassociated point sources close to the Galactic plane, which
are possibly related to limitations of the interstellar emission model and
missing extended sources.
