  The BICEP3 CMB Polarimeter is a small-aperture refracting telescope located
at the South Pole and is specifically designed to search for the possible
signature of inflationary gravitational waves in the Cosmic Microwave
Background (CMB). The experiment measures polarization on the sky by
differencing the signal of co-located, orthogonally polarized antennas coupled
to Transition Edge Sensor (TES) detectors. We present precise measurements of
the absolute polarization response angles and polarization efficiencies for
nearly all of BICEP3s $\sim800$ functioning polarization-sensitive detector
pairs from calibration data taken in January 2018. Using a Rotating Polarized
Source (RPS), we mapped polarization response for each detector over a full 360
degrees of source rotation and at multiple telescope boresight rotations from
which per-pair polarization properties were estimated. In future work, these
results will be used to constrain signals predicted by exotic physical models
such as Cosmic Birefringence.
