It would seem a third entry in the Plants vs. Zombies: Garden Warfare series is already in the works if an Amazon listing is to be believed.
Spotted by Reddit user codmycave, on the Amazon listing of the upcoming comic Plants vs. Zombies: Garden Warfare Volume 2, the description reads as:
Paul Tobin returns to write the official comic book bridge between video games PvZ: GW2 and PvZ: GW3 with Tim Lattie returning to deliver artwork
Developer Electronic Arts have yet to announce any plans for the second sequel in the series, however, with EA Play happening real soon in June, alongside E3, it is highly likely that any announcements about Plants vs. Zombies: Garden Warfare will happen there.