The AOT anime moves at such a glacial pace, and there's so long between seasons... I still haven't watched past S2, because after S1 I went and started reading the manga, and like three hours of reading put me well past even where season 2 ended. It's like every episode is three pages in the manga lol. Really compelling mysteries, but at this point I'm waiting until it's all done.
There aren't large discrepancies from what I remember, but they removed some stuff. Off the top of my head, for example, the 1st day Reiner invasion flashback is much longer in the manga from what I remember. The anime cut some stuff (I think some of Annie's involvement, in the anime they make it seem like she was chocked out, but iirc it wasn't the case in the manga)