So then, if you want to get in this game for the story, and are committed enough to play through all 3 previous entries, would I recommend you the 3 previous entries? Not really, the previous 3 entries are all flawed in one way or another. However Drakengard and Drakengard 3 are in my opinion the least flawed GAMEPLAY wise.
Drakengard and Drakengard 3 are odd ones, it's as if someone took Panzer Dragoon series and combined it with Dynasty Warriors while attaching RPG mechanics to it. For the most part it works, but it can get very monotonous specially if you happen to be overleveled. Thankfully the bossfights and boss events inject substance to the monotony.
Nier is the most varied one gameplay wise. However it has two great flaws, the AI and the weapon's balancing. So basically the AI is fucking dumb, it can easily get stuck or be kited into corners of the world map. Their move set lacks variety and they rarely dodge any of your hits. Now on to the weapons, boy oh boy.
So the problem with the weapons' balancing is the magic Words system and the fact that upgrades are too easy to obtain. Some weapons are a little harder to obtain, but you really don't have to go out of your way to get them either. So why are weapons so easy to upgrade? You only need cash to upgrade weapons. So why is it a problem? It's a problem because the game has enemies drop loot and trash you can sell almost constantly. There is plot reasons for this, which will give you some massive feels, but for the most part, gameplay wise it dumbs down the game severely.
During boss fights get ready for some crazy ass bullet hell style shenanigans, but once you figure out the exploit to them, I suggest for you not to use it. Because frankly it takes out A LOT from the experience. Or you can be a cheap faggot and completely break the one thing the game does competently.
Story wise Nier and Drakengard are really solid, but Drakengard 3 just feels like a crappy anime to be honest. It does add to the lore, but the acting in both releases, the music, and aesthetics are just blegh. Nier Automata also feels anime-esque, but it's balanced out with the mecha designs and the story being much more focused like Nier and Drakengard were.