Attack on Titan, Hajime Isayama's runaway hit manga, has been going strong since beginning serialization in 2009. An equally popular anime adaptation began its run in 2013 and, as of summer 2019, has aired three seasons. The fourth, and final, season will be released in 2020 while the manga is, according to Isayama, currently in its final stages. Naturally, fans are hoping that the parallel production of both will allow the dark fantasy anime's ending to be reasonably faithful -- if not completely -- to its source material.
History is on hope's side here, as unlike other much longer-running anime, Attack on Titan is a deliberately lean beast; even going so far as to take a four-year hiatus between Seasons One and Two for more chapters of the manga to be released. While this killed a little of its momentum, returning viewers in 2018 and 2019 were rewarded by an explosive third act split across two parts -- all killer and absolutely no pesky filler. Or, even worse, an invented storyline that couldn't compete with what was penned by the books' creator (cough cough, Game of Thrones, cough cough).
Anime-only fans will be pleased to know that they're not missing out on much by not reading the manga. There are no wild diversions to compare and contrast between the two iterations (not yet, at least). However, as is so often the case with page-to-screen adaptations, not everything is exact in its migration: characterization and timeline tweaks, scene-padding and a bit of exposition-skipping means that reading at least certain portions of the manga will be beneficial to those only familiar with the anime, while others simply offer a different perspective on particular scenes.
At the time of writing, Isayama's manga has reached over 120 chapters. While that may sound like a lot to those unfamiliar with Japanese comics, it's nowhere near as daunting as, say, sitting down to begin plowing through One Piece's 900+. In other words, getting up to speed with the Attack on Titan manga while you wait for the final season of the anime to be released -- which will probably be summer 2020 -- is totally doable.
For those that would prefer to just consolidate their knowledge, there's not much to be gained from reading over the chapters that Season 1 and 2 pull from. Season 1 covers Chapters 1-33 while Season 2 covers 34-50. The differences between them are fairly slight; there are name changes (the Colossal Titan is the "Colossus" while the Omni-Directional Mobility gear is the "Vertical-Maneuvering gear" in the manga) and some alternations to the way the big final fight between Eren and Annie in their Titan forms goes down. "The Female Titan" arc takes place between Chapters 19-34, if you want to seek out those changes.
The most notable differences are the order of certain events, with the anime choosing to bump things up or compress the timeline to better suit its own pacing. Sasha stans might also be disappointed with how little she features in the manga compared to the anime, in which the potato-chomping Scout's unexpected popularity with viewers nabbed her increased screentime.
While by no means a radical departure, Season 3 of Attack on Titan skims over quite a bit of exposition from the manga -- exposition which, as fans will know, is absolutely crucial to the second half of Isayama's bleak and bloody saga. This third season covers Chapters 51-90 of the manga and blows the lid on almost everything you've been dying to get answers to since seeing the Colossal Titan poke his head up over Wall Maria at the story's start.
Once again, you're not missing out on much by not reading the manga but there's frankly so much game-changing information to take in that, if you're looking for a good jumping-on point, this juncture is the best place to do it. Recommended reading is Chapters 51-70, the "Uprising" arc, which details the Reiss and Ackerman families' hidden history; Rod Reiss' desperate assault on Ovud District; Kenny's death and Historia's crowning. You can then skip to Chapters 86-90 -- halfway through the "Return to Shiganshina" arc -- for Grisha Jeager/Yeager's Marleyan childhood; the racial war between the Eldians and the Marlyeans and how Grisha obtained the Attack Titan power.
If you want to pick up where Season 3 left off, just go straight to Chapter 91 to prepare yourself for Attack on Titan's last season.
The Attack on Titan manga is available from Kodansha. The Attack on Titan anime will return for a fourth season in fall 2020, though the show's exact release date is unknown at this time. It's unknown at this time if Season 4 will be split into two halves.