Ultimately, you probably already know if you should watch the third season, because you'll (hopefully) have watched the second.
But, man oh man, I really gotta say, they really enjoy outclassing themselves every season.
If I could put it into a metaphor,
Season one is like a well done steak. Yes, it's hardly the best steak, but it's steak and it's good even if it's tough to get through.
Season two is like your regular, medium rare steak, you've made the steak about as good as it can get and you're going to enjoy every bite.
Season three is like, you went out and got the premium steak, you got some of that good stuff, A5 Kobe beef strip or something. And you've got just the right amount of seasoning and cooked it perfectly. Could you possibly have a better steak? Sure, maybe. But holy hot potatoes is that some fiiiine fuckin steak.
Edit, now give me an Afterglow anime and I can die happy.