A pre-alpha build is available on Google Play. You'll need a device running at least Android 6.0 with specs equivalent to or better than the Samsung Galaxy S7. A limited number of spots are available, though PopCap will open up more in phases.
Since this is an early version, you'll likely encounter bugs (of software rather than garden variety), and it isn't representative of how the full version of Plants vs. Zombies 3 will look. PopCap is seeking feedback on combat for now, and the studio may have to wipe out your progress as it makes changes. There's no formal release date as yet for the full game.