  Gaia Early Data Release 3 contains astrometry and photometry results for
about 1.8 billion sources based on observations collected by the ESA Gaia
satellite during the first 34 months of operations. This paper focuses on the
photometric content, describing the input data, the algorithms, the processing,
and the validation of the results. Particular attention is given to the quality
of the data and to a number of features that users may need to take into
account to make the best use of the EDR3 catalogue. The treatment of the BP and
RP background has been updated to include a better estimation of the local
background, and the detection of crowding effects has been used to exclude
affected data from the calibrations. The photometric calibration models have
also been updated to account for flux loss over the whole magnitude range.
Significant improvements in the modelling and calibration of the point and line
spread functions have also helped to reduce a number of instrumental effects
that were still present in DR2. EDR3 contains 1.806 billion sources with G-band
photometry and 1.540 billion sources with BP and RP photometry. The median
uncertainty in the G-band photometry, as measured from the standard deviation
of the internally calibrated mean photometry for a given source, is 0.2 mmag at
magnitude G=10 to 14, 0.8 mmag at G=17, and 2.6 mmag at G=19. The significant
magnitude term found in the Gaia DR2 photometry is no longer visible, and
overall there are no trends larger than 1 mmag/mag. Using one passband over the
whole colour and magnitude range leaves no systematics above the 1% level in
magnitude in any of the bands, and a larger systematic is present for a very
small sample of bright and blue sources. A detailed description of the residual
systematic effects is provided. Overall the quality of the calibrated mean
photometry in EDR3 is superior with respect to DR2 for all bands.
