It's even more different from Season 2 than that season had been from Season 1. It's not even really the same genre. This is great. It has revelations of a kind we'd never even imagined and it's taking SPOILER's characterisation in directions that would originally have been unthinkable. Also SPOILER. This will be a short review, because I'm trying to be spoiler-free in a story where pretty much everything is a spoiler by now.
Season 1 (eps.1-25) was an intense all-action zombie apocalypse, where the zombies (i.e. Titans) are up to 60 metres tall and just eat you whole. The human race's last remnants have been living inside a wall for a hundred years.
Season 2 (eps.26-37) expanded our views of Titans and turned some of them into characters. It blurs a lot of lines.
Season 3 Part 1 (eps.39-49) takes that a lot further. For starters, we don't meet any existing Titans. Instead, it's a season of politics. If you think that sounds dull, you haven't visited Trost District. The military police will torture prisoners to death. The First Interior Squad employ serial killers... and the Royal Government is worse than both of them.
This season isn't about Titans killing people. Humanity's doing that just fine all by itself.
After that, things change again. The secrets and changes of setting are so extreme that you might wonder if you've missed an episode. The most important person in the show becomes someone who's been dead since before ep.1. We see the biggest Titan yet... which gives us some surprising (and disgusting) real-world physics. How does a 120-metre Titan remain standing? Answer: it doesn't. Watch carefully and you'll see that when it first appears, it still has a face.
The season even has a tiny amount of humour! The childish, peurile (and sadistic) revenge in ep.43 is quite funny, while ep.49 is outright comedic in a "before the horror" way.
I didn't like the theme song at first, but it grew on me. It's the weakest yet, but at least it's not trying to be Season 1's theme music again. There's a bad battle music choice in ep.44, though. Rap. (But briefly.)
(Season 3 Part 2 aired in May-July 2019, for what it's worth. Part 1 came out on English-language DVD almost exactly a year after it started airing in Japan, so I'm expecting Part 2 to appear in May 2020.)
Tomoko says it's better than the equivalent manga chapters, incidentally, because they've compressed a story arc that in the original dragged on a bit. Apparently even the manga's creator thinks it's an improvement. Personally, though, I'm tempted to wonder if they might have over-compressed. I was okay, but I was still surprised by the amount of blindsiding right turns this season. Where is this place and how did we get there? Who are these people? Why is SPOILER saying all that? Answer: you've just got to roll with it.
It's still good, though. Frankly, it's improved the show. Season 1 was intense, but this gives you more to think about. Apparently the manga's getting close to the end, while we've been promised that the anime will end with Season 4 in 2020. Once we have everything, I might do a big rewatch.