  P\'egase.3 is a Fortran 95 code modeling the spectral evolution of galaxies
from the far-ultraviolet to submillimeter wavelengths. It also follows the
chemical evolution of their stars, gas and dust.
  For a given scenario (a set of parameters defining the history of mass
assembly, the star formation law, the initial mass function...), P\'egase.3
consistently computes the following:
  * the star formation, infall, outflow and supernova rates from 0 to 20 Gyr;
  * the stellar metallicity, the abundances of main elements in the gas and the
composition of dust;
  * the unattenuated stellar spectral energy distribution (SED);
  * the nebular SED, using nebular continua and emission lines precomputed with
code Cloudy (Ferland et al. 2017);
  * the attenuation in star-forming clouds and the diffuse interstellar medium,
by absorption and scattering on dust grains, of the stellar and nebular SEDs.
For this, the code uses grids of the transmittance for spiral and spheroidal
galaxies. We precomputed these grids through Monte Carlo simulations of
radiative transfer based on the method of virtual interactions;
  * the re-emission by grains of the light they absorbed, taking into account
stochastic heating.
  The main innovation compared to P\'egase.2 is the modeling of dust emission
and its evolution. The computation of nebular emission has also been entirely
upgraded to take into account metallicity effects and infrared lines.
  Other major differences are that complex scenarios of evolution (derived for
instance from cosmological simulations), with several episodes of star
formation, infall or outflow, may now be implemented, and that the detailed
evolution of the most important elements -- not only the overall metallicity --
is followed.
