Just buy The Story So Far.
Kingdom Hearts 3 was an awful disappointment. the only thing that i think was outright improved was the damn Gummi Ship bits, which i seriously hated in the earlier games, and hate more in this one. the use of Pixar worlds everywhere and only 1 or 2 Disney worlds is just horrible.
Kingdom Hearts 2 is to Kingdom Hearts as how Kingdom Hearts 3 should have been to Kingdom Hearts 2.
i mean, come on, Kingdom Hearts 2 took 3 years after the first one, and it's been 14 years since Kingdom Hearts 2! i expected KH3 to build on the Kingdom Hearts series as KH2 did, at the VERY least. instead, we had a few small improvements, a few big ones in ways that no one cared about, and some things that were removed or made worse (like Drive and Drive forms being replaced with Focus and Shotlock, which i always hated)
Kingdom Hearts: The Story So Far features every Kingdom Hearts game before the third one for 40$, and is a much, MUCH better experience than Kingdom Hearts 3, barring Chain Of Memories. The combat is more robust, you get a ton more to play because it has like 5 full games, and even if you've already played the games, it's always fun to replay.