  We use cosmic microwave background (CMB) temperature maps from the 500
deg$^{2}$ SPTpol survey to measure the stacked lensing convergence of galaxy
clusters from the Dark Energy Survey (DES) Year-3 redMaPPer (RM) cluster
catalog. The lensing signal is extracted through a modified quadratic estimator
designed to be unbiased by the thermal Sunyaev-Zel{'}dovich (tSZ) effect. The
modified estimator uses a tSZ-free map, constructed from the SPTpol 95 and 150
GHz datasets, to estimate the background CMB gradient. For lensing
reconstruction, we employ two versions of the RM catalog: a flux-limited sample
containing 4003 clusters and a volume-limited sample with 1741 clusters. We
detect lensing at a significance of 8.7$\sigma$(6.7$\sigma$) with the
flux(volume)-limited sample. By modeling the reconstructed convergence using
the Navarro-Frenk-White profile, we find the average lensing masses to be
$M_{200m}$ = ($1.62^{+0.32}_{-0.25}$ [stat.] $\pm$ 0.04 [sys.]) and
($1.28^{+0.14}_{-0.18}$ [stat.] $\pm$ 0.03 [sys.]) $\times\ 10^{14}\ M_{\odot}$
for the volume- and flux-limited samples respectively. The systematic error
budget is much smaller than the statistical uncertainty and is dominated by the
uncertainties in the RM cluster centroids. We use the volume-limited sample to
calibrate the normalization of the mass-richness scaling relation, and find a
result consistent with the galaxy weak-lensing measurements from DES
(Mcclintock et al. 2018).
