  Many scientific goals for the Dark Energy Survey (DES) require calibration of
optical/NIR broadband $b = grizY$ photometry that is stable in time and uniform
over the celestial sky to one percent or better. It is also necessary to limit
to similar accuracy systematic uncertainty in the calibrated broadband
magnitudes due to uncertainty in the spectrum of the source. Here we present a
"Forward Global Calibration Method (FGCM)" for photometric calibration of the
DES, and we present results of its application to the first three years of the
survey (Y3A1). The FGCM combines data taken with auxiliary instrumentation at
the observatory with data from the broad-band survey imaging itself and models
of the instrument and atmosphere to estimate the spatial- and time-dependence
of the passbands of individual DES survey exposures. "Standard" passbands are
chosen that are typical of the passbands encountered during the survey. The
passband of any individual observation is combined with an estimate of the
source spectral shape to yield a magnitude $m_b^{\mathrm{std}}$ in the standard
system. This "chromatic correction" to the standard system is necessary to
achieve sub-percent calibrations. The FGCM achieves reproducible and stable
photometric calibration of standard magnitudes $m_b^{\mathrm{std}}$ of stellar
sources over the multi-year Y3A1 data sample with residual random calibration
errors of $\sigma=5-6\,\mathrm{mmag}$ per exposure. The accuracy of the
calibration is uniform across the $5000\,\mathrm{deg}^2$ DES footprint to
within $\sigma=7\,\mathrm{mmag}$. The systematic uncertainties of magnitudes in
the standard system due to the spectra of sources are less than
$5\,\mathrm{mmag}$ for main sequence stars with $0.5<g-i<3.0$.
