  We present the first cosmological parameter constraints using measurements of
type Ia supernovae (SNe Ia) from the Dark Energy Survey Supernova Program
(DES-SN). The analysis uses a subsample of 207 spectroscopically confirmed SNe
Ia from the first three years of DES-SN, combined with a low-redshift sample of
122 SNe from the literature. Our "DES-SN3YR" result from these 329 SNe Ia is
based on a series of companion analyses and improvements covering SN Ia
discovery, spectroscopic selection, photometry, calibration, distance bias
corrections, and evaluation of systematic uncertainties. For a flat LCDM model
we find a matter density Omega_m = 0.331 +_ 0.038. For a flat wCDM model, and
combining our SN Ia constraints with those from the cosmic microwave background
(CMB), we find a dark energy equation of state w = -0.978 +_ 0.059, and Omega_m
= 0.321 +_ 0.018. For a flat w0waCDM model, and combining probes from SN Ia,
CMB and baryon acoustic oscillations, we find w0 = -0.885 +_ 0.114 and wa =
-0.387 +_ 0.430. These results are in agreement with a cosmological constant
and with previous constraints using SNe Ia (Pantheon, JLA).
