Now 20 years after the discovery of the accelerating universe, distance measurements from Type Ia supernovae over a large span in redshift remain a vital tool in constraining models for cosmic acceleration and dark energy. There has been much effort focused on generating larger and more precise datasets with good control of systematic uncertainties. The Dark Energy Survey Supernova Program (DES-SN) is the most recent such effort and has doubled the number of cosmologically viable SNe. In addition, the Dark Energy Survey Gravitational Waves program (DES-GW) identified the optical counterpart of the Binary Neutron star merger GW170817 and facilitated the first ever ``standard siren'' measurement of $H_0$; a new cosmic probe independent of the local distance ladder. In this thesis I present the development of the photometric pipeline, detailed cosmological analyses and evaluation of systematic uncertainties, Hubble Diagram, cosmological parameter constraints, and tests of various models of dark energy for 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