Dark Energy Survey Year 3 results: cosmology from combined galaxy
  clustering and lensing -- validation on cosmological simulations
  We present a validation of the Dark Energy Survey Year 3 (DES Y3)
$3\times2$-point analysis choices by testing them on Buzzard v2.0, a new suite
of cosmological simulations that is tailored for the testing and validation of
combined galaxy clustering and weak lensing analyses. We show that the Buzzard
v2.0 simulations accurately reproduce many important aspects of the DES Y3
data, including photometric redshift and magnitude distributions, and the
relevant set of two-point clustering and weak lensing statistics. We then show
that our model for the $3\times2$-point data vector is accurate enough to
recover the true cosmology in simulated surveys assuming the true redshift
distributions for our source and lens samples, demonstrating robustness to
uncertainties in the modeling of the non-linear matter power spectrum,
non-linear galaxy bias and higher-order lensing corrections. Additionally, we
demonstrate for the first time that our photometric redshift calibration
methodology, including information from photometry, spectroscopy, clustering
cross-correlations, and galaxy-galaxy lensing ratios, is accurate enough to
recover the true cosmology in simulated surveys in the presence of realistic
photometric redshift uncertainties.
