  We present the analysis underpinning the measurement of cosmological
parameters from 207 spectroscopically classified type Ia supernovae (SNe Ia)
from the first three years of the Dark Energy Survey Supernova Program
(DES-SN), spanning a redshift range of 0.017<$z$<0.849. We combine the DES-SN
sample with an external sample of 122 low-redshift ($z$<0.1) SNe Ia, resulting
in a "DES-SN3YR" sample of 329 SNe Ia. Our cosmological analyses are blinded:
after combining our DES-SN3YR distances with constraints from the Cosmic
Microwave Background (CMB; Planck Collaboration 2016), our uncertainties in the
measurement of the dark energy equation-of-state parameter, $w$, are .042
(stat) and .059 (stat+syst) at 68% confidence. We provide a detailed systematic
uncertainty budget, which has nearly equal contributions from photometric
calibration, astrophysical bias corrections, and instrumental bias corrections.
We also include several new sources of systematic uncertainty. While our sample
is <1/3 the size of the Pantheon sample, our constraints on $w$ are only larger
by 1.4$\times$, showing the impact of the DES SN Ia light curve quality. We
find that the traditional stretch and color standardization parameters of the
DES SNe Ia are in agreement with earlier SN Ia samples such as Pan-STARRS1 and
the Supernova Legacy Survey. However, we find smaller intrinsic scatter about
the Hubble diagram (0.077 mag). Interestingly, we find no evidence for a Hubble
residual step ( 0.007 $\pm$ 0.018 mag) as a function of host galaxy mass for
the DES subset, in 2.4$\sigma$ tension with previous measurements. We also
present novel validation methods of our sample using simulated SNe Ia inserted
in DECam images and using large catalog-level simulations to test for biases in
our analysis pipelines.
