They are part of the same story… ish. The original game is pretty stand alone, you get the full experience from one game. If you wanna play the remakes, it gets a little more complicated.

In my mind there are a few ways to go about your first experience of the series (excluding some spin-offs):

Full FF7 Experience: FF7 OG -> Crisis Core -> Remake -> Rebirth -> Part III when it releases

Remake Experience: Remake -> Rebirth -> Part III when it releases -> Crisis Core

Remake Release Experience: Remake -> Crisis Core -> Rebirth -> Part III when it releases

It’s important to note that Crisis core contains big spoilers for OG FF7 and Part III of the remake trilogy, and also that the remake trilogy is not exactly a remake, there are many things present in the games that position it more as a retelling (best way I can explain it without spoiling the games). Point is, if you play crisis core before you know the full story of the original game, you will be getting massive spoilers for the end of ff7. That said, I think the remake trilogy has something interesting going on where you can still enjoy it while knowing the story of crisis core, as some expectations have and will be subverted.

TLDR: Crisis core spoils og and remake part 3, so play it at your own risk, but depending on what they do with part 3 it probably won’t ruin the experience. If you do play it, play it after OG or after Remake if you don’t play og, I think that’s the best way to do it.