  $ $Low density cosmic voids gravitationally lens the cosmic microwave
background (CMB), leaving a negative imprint on the CMB convergence $\kappa$.
This effect provides insight into the distribution of matter within voids, and
can also be used to study the growth of structure. We measure this lensing
imprint by cross-correlating the Planck CMB lensing convergence map with voids
identified in the Dark Energy Survey Year 3 data set, covering approximately
4,200 deg$^2$ of the sky. We use two distinct void-finding algorithms: a 2D
void-finder which operates on the projected galaxy density field in thin
redshift shells, and a new code, Voxel, which operates on the full 3D map of
galaxy positions. We employ an optimal matched filtering method for
cross-correlation, using the MICE N-body simulation both to establish the
template for the matched filter and to calibrate detection significances. Using
the DES Y3 photometric luminous red galaxy sample, we measure $A_\kappa$, the
amplitude of the observed lensing signal relative to the simulation template,
obtaining $A_\kappa = 1.03 \pm 0.22$ ($4.6\sigma$ significance) for Voxel and
$A_\kappa = 1.02 \pm 0.17$ ($5.9\sigma$ significance) for 2D voids, both
consistent with $\Lambda$CDM expectations. We additionally invert the 2D
void-finding process to identify superclusters in the projected density field,
for which we measure $A_\kappa = 0.87 \pm 0.15$ ($5.9\sigma$ significance). The
leading source of noise in our measurements is Planck noise, implying that
future data from the Atacama Cosmology Telescope (ACT), South Pole Telescope
(SPT) and CMB-S4 will increase sensitivity and allow for more precise
measurements.
