  We present spectroscopy from the first three seasons of the Dark Energy
Survey Supernova Program (DES-SN). We describe the supernova spectroscopic
program in full: strategy, observations, data reduction, and classification. We
have spectroscopically confirmed 307 supernovae, including 251 type Ia
supernovae (SNe Ia) over a redshift range of $0.017 < z < 0.85$. We determine
the effective spectroscopic selection function for our sample, and use it to
investigate the redshift-dependent bias on the distance moduli of SNe Ia we
have classified. We also provide a full overview of the strategy, observations,
and data products of DES-SN, which has discovered 12,015 likely supernovae
during these first three seasons. The data presented here are used for the
first cosmology analysis by DES-SN ('DES-SN3YR'), the results of which are
given in DES Collaboration (2018a).
