If you've never played any Kingdom Hearts game you are going to be absolutely lost with the story in Kingdom Hearts 3.
I played the first 2 and had to watch a half hour YouTube video to fill in the gaps from the spin off games that I didn't play to get a handle on the story.
Basically, if you only played the first 2 you're going to be lost, so watch a YouTube video to help.
If you never played any of them, nothing is going to make sense to you at all and it's a shame, because the game is fun, and is incredibly gorgeous.
They did an TERRIBLE job with this series in terms of the story and the releases.
To fully understand the story you would have had to play all these games which launched on different platforms over the span of 17 years.
You have releases that originally came out on: PS2, Game Boy Advance, Nintendo DS, PSP, Nintendo 3DS, and now PS4/XB1.
They did release HD versions of each game later. I own them, and tried to force myself to play them but quickly found myself without the interest in trucking through what felt to me like incredibly dated games with too many monotonous sections for me to truck through just to hope to understand a story that I knew at the end I would still be confused about.
The story involves multiple versions of each character (good, bad, neutral) as well as past, future, and present versions of themselves, which....can travel through time.
To give you a slight bit of hope....if you're really interested in playing the game still even though you never played any of the other ones, just know that the overarching plot is simple enough to follow: The good guys are trying to save their friends (Some of which are lost, some of which may have been capture by the bad guys, some which may not even remember who they really are due to the different versions of themselves I previously mentioned). The bad guys want to rule the world (and yes, they have alternate versions of themselves that exist in different timelines and in the same timelines, all with different names depending on their appearance in each timeline). So the plot is simple enough to follow, but you won't ever have a clue of who is who, and why they care about anything.