David Harbour (Chief Jim Hopper) had already indicated that season three probably wouldn’t arrive until 2019. Netflix’s vice president of original programming Cindy Holland then confirmed that season three would not be released until summer 2019, saying however that the delay will be “worth the wait”.
“[Creators] The Duffer Brothers and [producer] Shawn Levy, they understand the stakes are high,” she said. “They want to deliver something bigger and better than last year. I think it’s going to be a fantastic season. It will be worth the wait.”
On 10 November 2018, David Harbour confirmed that he had finished filming – and that he was looking forward to shaving off his moustache.
The summer 2019 release ties in with the show’s initial ‘mall teaser’, which first hinted at a summer release.
In September 2018, executive producer and director Shawn Levy said during the Emmys that “cinematically, it’s our biggest season yet”, which perhaps goes some way to explain the additional time required to film season three.