I cannot recommend this series enough. All three seasons have had intricate plots, internally consistent mysteries, and characters you want to cheer for. It also tends to play on my emotions. It is not, however, kid appropriate--there are some genuinely terrifying moments, and the aforementioned gore.
The third season moves logically from the previous ones. The Goonies-like pack of friends is getting older and dealing with their changing relationships while their older siblings are dealing with issues that new adults face. Meanwhile, the terror that lurks in Hawkins begins a new, deeply disturbing, plan.
Oddly, my favorite parts of the show are not the main set-piece moments (although those are awesome), but the quiet moments where the characters bicker and bond with one another. Like season 2, a couple of new characters get some time in the spotlight, and they each add new layers to the overall group dynamics. They are certainly set up for season 4, and I again find myself waiting eagerly for them to start production so I can binge watch that one too.