  X-ray polarization is a powerful tool to investigate the geometry of
accreting material around black holes, allowing independent measurements of the
black hole spin and orientation of the innermost parts of the accretion disk.
We perform the X-ray spectro-polarimetric analysis of an X-ray binary system in
the Large Magellanic Cloud, LMC X-3, that hosts a stellar-mass black hole,
known to be persistently accreting since its discovery. We report the first
detection of the X-ray polarization in LMC X-3 with the Imaging X-ray
Polarimetry Explorer, and find the average polarization degree of 3.2% +- 0.6%
and a constant polarization angle -42 deg +- 6 deg over the 2-8 keV range.
Using accompanying spectroscopic observations by NICER, NuSTAR, and the Neil
Gehrels Swift observatories, we confirm previous measurements of the black hole
spin via the X-ray continuum method, a ~ 0.2. From polarization analysis only,
we found consistent results with low black-hole spin, with an upper limit of a
< 0.7 at a 90% confidence level. A slight increase of the polarization degree
with energy, similar to other black-hole X-ray binaries in the soft state, is
suggested from the data but with a low statistical significance.
