  We present the photometric calibration of the twelve optical passbands for
the Javalambre Photometric Local Universe Survey (J-PLUS) third data release
(DR3), comprising 1642 pointings of two square degrees each. We selected nearly
1.5 million main sequence stars with a signal-to-noise ratio larger than ten in
the twelve J-PLUS passbands and available low-resolution (R = 20-80) spectrum
from the blue and red photometers (BP/RP) in Gaia DR3. We compared the
synthetic photometry from BP/RP spectra with the J-PLUS instrumental
magnitudes, after correcting for the magnitude and color terms between both
systems, to obtain an homogeneous photometric solution for J-PLUS. To
circumvent the current limitations in the absolute calibration of the BP/RP
spectra, the absolute color scale was derived using the locus of 109 white
dwarfs closer than 100 pc with a negligible interstellar extinction. Finally,
the absolute flux scale was anchored to the Panoramic Survey Telescope and
Rapid Response System (Pan-STARRS) photometry in the r band. The precision of
the J-PLUS photometric calibration, estimated from duplicated objects observed
in adjacent pointings and by comparison with the spectro-photometric standard
star GD 153, is ~12 mmag in u, J0378, and J0395; and ~7 mmag in J0410, J0430,
g, J0515, r, J0660, i, J0861, and z. The estimated accuracy in the calibration
along the surveyed area is better than 1% for all the passbands. The Gaia BP/RP
spectra provide a high-quality, homogeneous photometric reference in the
optical range across the full-sky, in spite of their current limitations as an
absolute reference. The calibration method for J-PLUS DR3 reaches an absolute
precision and accuracy of 1% in the twelve optical filters within an area of
3284 square degrees.
