Herschel-ATLAS Data Release III: Near-infrared counterparts in the South
  Galactic Pole field -- Another 100,000 submillimetre galaxies
  In this paper we present the third data release (DR3) of the Herschel
Astrophysical Terahertz Large Area Survey (H-ATLAS). We identify likely
near-infrared counterparts to submillimetre sources in the South Galactic Pole
(SGP) field using the VISTA VIKING survey. We search for the most probable
counterparts within 15 arcsec of each Herschel source using a probability
measure based on the ratio between the likelihood the true counterpart is found
close to the submillimetre source and the likelihood that an unrelated object
is found in the same location. For 110 374 (57.0$\%$) sources we find galaxies
on the near-infrared images where the probability that the galaxy is associated
to the source is greater than 0.8. We estimate the false identification rate to
be 4.8$\%$, with a probability that the source has an associated counterpart on
the VIKING images of 0.835$\pm$0.009. We investigate the effects of
gravitational lensing and present 41 (0.14 deg$^{-2}$) candidate lensed systems
with observed flux densities > 100 mJy at 500 $\mu$m. We include in the data
release a probability that each source is gravitationally lensed and discover
an additional 5 923 sources below 100 mJy that have a probability greater than
0.94 of being gravitationally lensed. We estimate that $\sim$ 400 - 1 000
sources have multiple true identifications in VIKING based on the similarity of
redshift estimates for multiple counterparts close to a Herschel source. The
data described in this paper can be found at the H-ATLAS website.
