Turning the Season 2 has just started, the Duffer brothers said they had worked on Season 3.
Information confirmed by Shawn Levy, Producer and director of the series, in an interview with the specialized site Collider " We are very optimistic about the future of series, so we already anticipate the events. We do not want to be caught out and have to make up stories a day before to write and work on it, so it is clearly optimistic and we started to think about it upstream. "
However, although the first season was a huge success, it will take the exit and see the success that should meter Season 2 for a season of confirmation 3 side Netflix.
But in view of the quality of the first season, we are relatively confident for the next two seasons.