Fuck. Now I can't play the game until I play Birth By Sleep. I thought maybe I could get away with it, but there's just no way now.
Oh yeah, you should at the very least watch the story from that game before playing Kingdom Hearts 3.
It's where the main villain is introduced, and his background and motivation is shown, and it's the lead up to the current situation in Kingdom Hearts 3.
I don't know if they'll recap it in KH3, but a lot of crucial plot elements in Kingdom Hearts 3 are the way they are because of what happened in BBS. I'd even go as far as to say that the events of Kingdom Hearts 2 are more skippable than BBS's story, when it comes to enjoying KH3.
Because Kingdom Hearts 2 is more like a conclusion of the first section of the story, wheres BBS introduced a new chapter, that will now conclude in KH3.
First story: Kingdom Hearts 1 > Chain of Memories > 358/2 Days > Kingdom Hearts 2.
Second story: Birth By Sleep >Coded (Only the secret ending, really) > Dream Drop Distance > Kingdom Hearts 3.
Birth By Sleep has my favorite battle mechanics (cooldown system for skills, that can be reduced by using more skills, encouraging more frequent usage of skills) so I'd say it'd be fun to play, if you are at all into Kingdom Hearts. It'll probably pale in comparison to KH 3 though. But if you don't want to play BBS, at least watch the story on youtube is my recommendation.
Anyway, I really didn't want to get my hopes up about this character being playable in the main game, due to what we see in some KH3 trailers. Maybe in a DLC, I thought, but I'm very glad to see I was wrong about that.