I'll probably go for KH:TSSF then, that sounds like it would be the easiest way. Is χ a game you've played? Is it worth playing? Coded is not worth playing, but I might dig it out and finish it after I beat BBS, I was pretty close.
Yeah, that one sounds like the best deal.
I have played X actually. If it's worth playing? Well, it's a mobile gacha game, and it plays completely different from any other KH game. It's not something I'd recommend someone to play for the story, but rather if you happen to enjoy things like collecting your favorite characters, and the gameplay mechanics of the game.
You use in-game currency to get random characters and hope you get lucky to get the ones you want. Though the characters are usually guaranteed after a certain number of tries, which makes it possible for people to play and get most of the characters they want even without spending real money on the game (which I don't).
Here's what it looks like in action.
It can be nice for killing a few minutes here and there, and there are some interesting strategies for completing some fights in the game.
That said, my interest in it has waned a lot, to the point where I now just log in for the daily log in bonus (in-game currency), and save them for when a character I'm interested in shows up. I don't really play it any more. But that could change if they add some interesting new modes.
It's free to download, so you could always give it a try and see if it's something you'd want to keep playing for a while.
The story may be worth knowing though, whether you get it from playing the game or looking at the videos featured in the PS4 game KH:TSSF. I say 'may be' because they may summarize the important bits in KH3.
Some are directly tied to the plot of KH 3, such as the box we see in one of the KH3 trailers that the villain is after:
KH X takes place (I believe) hundreds of years before KH1, and it introduces some characters that will be important for KH3's plot, I'm sure.
It also shows the origin of one of the Organisation XIII members, whom I'm still confused as to why that one has been alive for so many years. Since nothing in the other KH games suggested this particular member of the Organisation was special in any way.
One thing to note though is that KHUX's story, including the part about the Org XIII member I mentioned, still hasn't concluded. (They add more story every now and then through updates.) Which is interesting, since KH3 will be out in a few days. So I guess I'll get most of my answers about these characters in KH3 first?