The Solar Wind ANisotropies (SWAN) all-sky hydrogen Lyα camera on the SOlar and Heliospheric Observer satellite observed the hydrogen coma of comet C/2017 S3 (PanSTARRS) for the last month of its activity from 2018 July 4 to August 4 and what appears to have been its final disintegration just 11 days before its perihelion on August 15. The hydrogen coma indicated water production had a small outburst on July 8 at a heliocentric distance of 1.1 au and then a much larger one on July 20 at 0.8 au. Over the following two weeks the water production dropped by more than a factor of 10 after which it was no longer detectable. The behavior is reminiscent of comet C/1999 S4 (LINEAR) in 2000, which had a few small outbursts on its inbound orbit and a major outburst at a heliocentric distance of about 0.8 au, which was close to its perihelion, followed by its complete disintegration that was documented by several sets of observations including SWAN. C/2017 S3 (PanSTARRS), however, had a much larger water production rate than C/1999 S4 (LINEAR). Here we estimate the size of the nucleus of C/2017 S3 just before its final outburst and apparent disintegration were estimated using the total amount of water produced during its last weeks for a range of values of the refractory/ice ratio in the nucleus. We also determine the size distribution of the disintegrating particles as the comet faded.