  A simple cosmological model with only six parameters (matter density, Omega_m
h^2, baryon density, Omega_b h^2, Hubble Constant, H_0, amplitude of
fluctuations, sigma_8, optical depth, tau, and a slope for the scalar
perturbation spectrum, n_s) fits not only the three year WMAP temperature and
polarization data, but also small scale CMB data, light element abundances,
large-scale structure observations, and the supernova luminosity/distance
relationship. Using WMAP data only, the best fit values for cosmological
parameters for the power-law flat LCDM model are (Omega_m h^2, Omega_b h^2, h,
n_s, tau, sigma_8) = 0.1277+0.0080-0.0079, 0.02229+-0.00073, 0.732+0.031-0.032,
0.958+-0.016, 0.089+-0.030, 0.761+0.049-0.048). The three year data
dramatically shrink the allowed volume in this six dimensional parameter space.
Assuming that the primordial fluctuations are adiabatic with a power law
spectrum, the WMAP data_alone_ require dark matter, and favor a spectral index
that is significantly less than the Harrison-Zel'dovich-Peebles scale-invariant
spectrum (n_s=1, r=0). Models that suppress large-scale power through a running
spectral index or a large-scale cut-off in the power spectrum are a better fit
to the WMAP and small scale CMB data than the power-law LCDM model; however,
the improvement in the fit to the WMAP data is only Delta chi^2 = 3 for 1 extra
degree of freedom. The combination of WMAP and other astronomical data yields
significant constraints on the geometry of the universe, the equation of state
of the dark energy, the gravitational wave energy density, and neutrino
properties. Consistent with the predictions of simple inflationary theories, we
detect no significant deviations from Gaussianity in the CMB maps.
