Three years of Wilkinson Microwave Anisotropy Probe (WMAP) data have now been analyzed. The 3 year data analysis is improved over that of the first-year WMAP publications by better models of the instrument characteristics, improved map making and a complete polarization analysis. The 3-year full-sky maps of intensity and polarization in 5 microwave bands are used test cosmological and galactic foreground models. With the higher sensitivity and polarization information, a Lambda Cold Dark Matter (LCDM) model remains an excellent fit to the data with parameters consistent with the first-year publications. The polarization maps have improved the determination of the optical depth of scattering due to reionization and reduced the degeneracy with other parameters, particularly the primordial spectral index of fluctuations. WMAP data combined with those of small angular-scale CMB experiments, an independent measure of the Hubble constant and large-scale structure and supernova results, improve the constraints on the dark energy equation of state, the total curvature, and the sum of the neutrino masses.