I've finally finished watching season 3 as well. What a ride!
Most of it has already been mentioned, so I'm not going to unnecessarily babble on about it. I agree with most of you about the weird "Neverending Story" sing-along in the middle of a very tense and pivotal moment, though. What was that about, and why?
Also, about the ending: I think "the American" is obviously meant to be Hopper. It still can turn out to be a red herring, especially if Hoppers' actor turns out to be too busy in the future, but I guess their plan is to have Hopper in place near the other location connected to the Upside Down, to start season 4 from there.
Speaking of which, is anyone interested in speculating about season 4 already?
We've established (https://cake.co/c/grRxX75/Qb9fN9j) that season 3 takes place in July 1985, and a three-month jump at the end of the season takes us to fall of that year. Add some more time, and I expect season 4 to take place in 1986. Considering that all three seasons so far had some connection to a different holiday season and to some extent season of the year (Christmas/Winter, Halloween/Fall, 4th of July/Summer), I wouldn't be surprised if season 4 did the same and be set around Easter/Spring.
Easter 1986 happened at the end of March - and from there, it's only four more weeks to one very defining moment of that year. With Hopper and some Demogorgons in a prison in Kamtschatka, I fully expect Hopper to be shuffled around the USSR for the season (revisiting those 1980s action/buddy movies set in the USSR), ending up in a military facility / secret lab near Pripyat, current-day Ukraine from where he is rescued back to the US by El's regained powers. Of course, this event will wreak havoc and cause the destruction of the Soviet facility on April 26, 1986, which will be covered up as a nuclear accident: