Conley, A.Guy, J.Sullivan, M.Regnault, N.Astier, P.Balland, C.Basa, S.Carlberg, R. G.Fouchez, D.Hardin, D.Hook, I. M.Howell, D. A.Pain, R.Palanque-Delabrouille, N.Perrett, K. M.Pritchet, C. J.Rich, J.Ruhlmann-Kleider, V.Balam, D.Baumont, S.Ellis, R. S.Fabbro, S.Fakhouri, H. K.Fourmanoit, N.González-Gaitán, S.Graham, M. L.Hudson, M. J.Hsiao, E.Kronborg, T.Lidman, C.Mourao, A. M.Neill, J. D.Perlmutter, S.Ripoche, P.Suzuki, N.Walker, E. S.2025-05-302025-05-300067-0049http://www.scopus.com/inward/record.url?scp=78650988595&partnerID=8YFLogxKhttps://hdl.handle.net/1885/733755082We combine high-redshift Type Ia supernovae from the first three years of the Supernova Legacy Survey (SNLS) with other supernova (SN) samples, primarily at lower redshifts, to form a high-quality joint sample of 472 SNe (123 low-z, 93 SDSS, 242 SNLS, and 14 Hubble Space Telescope). SN data alone require cosmic acceleration at >99.999% confidence, including systematic effects. For the dark energy equation of state parameter (assumed constant out to at least z = 1.4) in a flat universe, we find w = -0.91-0.20+0.16 (stat)+0.07-0.14 (sys) from SNe only, consistent with a cosmological constant. Our fits include a correction for the recently discovered relationship between hostgalaxy mass and SN absolute brightness. We pay particular attention to systematic uncertainties, characterizing them using a systematic covariance matrix that incorporates the redshift dependence of these effects, as well as the shape-luminosity and color-luminosity relationships. Unlike previous work, we include the effects of systematic terms on the empirical light-curve models. The total systematic uncertainty is dominated by calibration terms. We describe how the systematic uncertainties can be reduced with soon to be available improved nearby and intermediate-redshift samples, particularly those calibrated onto USNO/SDSS-like systems.enCosmological parametersCosmology: observationsSupernovae: general201110.1088/0067-0049/192/1/178650988595