Dark Energy Survey Year 3 Results: Clustering Redshifts -- Calibration
  of the Weak Lensing Source Redshift Distributions with redMaGiC and
  BOSS/eBOSS
  We present the calibration of the Dark Energy Survey Year 3 (DES Y3) weak
lensing source galaxy redshift distributions $n(z)$ from clustering
measurements. In particular, we cross-correlate the weak lensing (WL) source
galaxies sample with redMaGiC galaxies (luminous red galaxies with secure
photometric redshifts) and a spectroscopic sample from BOSS/eBOSS to estimate
the redshift distribution of the DES sources sample. Two distinct methods for
using the clustering statistics are described. The first uses the clustering
information independently to estimate the mean redshift of the source galaxies
within a redshift window, as done in the DES Y1 analysis. The second method
establishes a likelihood of the clustering data as a function of $n(z)$, which
can be incorporated into schemes for generating samples of $n(z)$ subject to
combined clustering and photometric constraints. Both methods incorporate
marginalisation over various astrophysical systematics, including magnification
and redshift-dependent galaxy-matter bias. We characterise the uncertainties of
the methods in simulations; the first method recovers the mean $z$ of
tomographic bins to RMS (precision) of $\sim 0.014$. Use of the second method
is shown to vastly improve the accuracy of the shape of $n(z)$ derived from
photometric data. The two methods are then applied to the DES Y3 data.
