Yes, it’s more of more of the same. Same baddies (mostly), same skills (that again need re-learning), same silly characters to interact with. But! Densetsu no Stafi 3 adds something new – a second, female, starfish. If my Japanese is correct, she’s Stafi’s sister Stapi.
At various times in the game you swap between the two starfish, sometimes at will, other times it is forced due to Events That Happen. Although similar, they have a couple of different skills. Notably, Stapi can wall-jump and crawl, so that’s one better than Metroid.
It adds quite a bit to the game, especially in a sections where you have to use each character’s skills to help the other character who in turn helps you. That said, this is really feels more of an extension of the series than a completely new game. If it ain’t broke, though. Oh! Except there is another thing which is new – WARIO. Yeah, for some reason Wario is in some of the levels and you have to guide him around. Weird.
There is a Densetsu no Stafi 4, which I own on the DS, but it’s a shame there’s no DS games on the Switch so I can carry on the series there.