>>713392632
>official way
Buy the 1.5, .5, and 2.8 collections (NOT on Switch because it's is streamed/cloud), which contain the important games plus movies of the unimportant ones. Then you can play Kingdom Hearts 3.
>fan method
Play 1.5 for the PS3 (or 1 for the PS2) for the original & best game
Play Chain of Memories on the GameBoy Advance (PS2/enhanced version is trickier due to 3D fights)
Play 2.5 for Kingdom Hearts 2 Final Mix (or just regular 2 on the PS2).
Read the Kingdom Hearts 358/2 manga
Ignore everything else related to the franchise.