  We present global radiative transfer models for heated relativistic jets. The
simulations include all relevant radiative processes, starting deep in the
opaque zone and following the evolution of radiation to and beyond the
photosphere of the jet. The transfer models are compared with three gamma-ray
bursts GRB 990123, GRB 090902B, and GRB 130427A, which have well-measured and
different spectra. The models provide good fits to the observed spectra in all
three cases, and we obtain estimates for the jet magnetization parameter
$\varepsilon_{\rm B}$ and the Lorentz factor $\Gamma$. In the small sample of
three bursts, $\varepsilon_{\rm B}$ varies between 0.01 and 0.05, and $\Gamma$
varies between 400 and 1200.
