  The paper presents the third data release of Hyper Suprime-Cam Subaru
Strategic Program (HSC-SSP), a wide-field multi-band imaging survey with the
Subaru 8.2m telescope. HSC-SSP has three survey layers (Wide, Deep, and
UltraDeep) with different area coverages and depths, designed to address a wide
array of astrophysical questions. This third release from HSC-SSP includes data
from 278 nights of observing time and covers about 670 square degrees in all
five broad-band filters at the full depth ($\sim26$~mag at $5\sigma$) in the
Wide layer. If we include partially observed area, the release covers 1,470
square degrees. The Deep and UltraDeep layers have $\sim80\%$ of the originally
planned integration times, and are considered done, as we have slightly changed
the observing strategy in order to compensate for various time losses. There
are a number of updates in the image processing pipeline. Of particular
importance is the change in the sky subtraction algorithm; we subtract the sky
on small scales before the detection and measurement stages, which has
significantly reduced false detections. Thanks to this and other updates, the
overall quality of the processed data has improved since the previous release.
However, there are limitations in the data (for example, the pipeline is not
optimized for crowded fields), and we encourage the user to check the quality
assurance plots as well as a list of known issues before exploiting the data.
The data release website is https://hsc-release.mtk.nao.ac.jp/.
