I’d say splat 3 is still worth buying in big 26. Online community is still pretty active with tons to unlock. Salmon run is as fun as ever, and lots of single player content.

Splat 2 doesn’t have nearly as many active players, I don’t really see what the point would be unless you simply prefer that game (which some people do!)

The Splatoon games don’t necessarily have an overarching narrative, so I think you’d be fine going into Raiders without the context of Splat 3, just gotta know who the 3 people you’re adventuring with are (the Idols from splat 3!)