Shawn Levy, series producer and director for “Stranger Things”, has said that filming for season 3 was beginning today. This season will drop back down to 8 episodes, the same as season 1 – season 2 consisted of 10.
Levy also reaffirmed that there will be a 4th season and possibly another after that:
I mean I will say we have a good sense of stuff that happens in Season Four. Season Four is definitely happening. There’s very much the possibility of a season beyond that one, that’s currently undecided.