The upcoming Sony a7S III is expected to be announced in the second half of 2019. According to previous interview with Sony manager, upcoming Sony a7S III may feature with 10bit 4:2:2 and 4K video at 60p. And a trusted source also told us that Sony a7S III could feature with a new 15MP full frame sensor, while current a7S II has a 12.2MP full frame sensor.
The recently leaked Sony new CMOS sensor IMX521 is a high speed, high sensitivity Quad Bayer 15MP BSI FF sensor. So this sensor may used on upcoming Sony a7S III. Stay tuned.