More info here: https://ww2.kqed.org/science/2017/09/15/ca-legislature-clean-cars-can-keep-carpool-lane-access/
Gist of it…
Stickers issued to new cars after January 1, 2019 are valid for three full years and then until January 1 of their fourth year.
Drivers issued stickers in 2017 and 2018 will be able to apply for a new sticker in 2019 that is valid until January 1, 2022.
Stickers issued before January 1, 2017 will expire on January 1, 2019