  We present cosmological constraints from a joint analysis of type Ia
supernova (SN Ia) observations obtained by the SDSS-II and SNLS collaborations.
The data set includes several low-redshift samples (z<0.1), all 3 seasons from
the SDSS-II (0.05 < z < 0.4), and 3 years from SNLS (0.2 <z < 1) and totals
\ntotc spectroscopically confirmed type Ia supernovae with high quality light
curves. We have followed the methods and assumptions of the SNLS 3-year data
analysis except for the following important improvements: 1) the addition of
the full SDSS-II spectroscopically-confirmed SN Ia sample in both the training
of the SALT2 light curve model and in the Hubble diagram analysis (\nsdssc
SNe), 2) inter-calibration of the SNLS and SDSS surveys and reduced systematic
uncertainties in the photometric calibration, performed blindly with respect to
the cosmology analysis, and 3) a thorough investigation of systematic errors
associated with the SALT2 modeling of SN Ia light-curves. We produce
recalibrated SN Ia light-curves and associated distances for the SDSS-II and
SNLS samples. The large SDSS-II sample provides an effective, independent,
low-z anchor for the Hubble diagram and reduces the systematic error from
calibration systematics in the low-z SN sample. For a flat LCDM cosmology we
find Omega_m=0.295+-0.034 (stat+sys), a value consistent with the most recent
CMB measurement from the Planck and WMAP experiments. Our result is 1.8sigma
(stat+sys) different than the previously published result of SNLS 3-year data.
The change is due primarily to improvements in the SNLS photometric
calibration. When combined with CMB constraints, we measure a constant
dark-energy equation of state parameter w=-1.018+-0.057 (stat+sys) for a flat
universe. Adding BAO distance measurements gives similar constraints:
w=-1.027+-0.055.
