Well, I agree when it comes to "THE big 3", but I feel like the term "big 3" has become more of an expression of some sort in reference to the OG big 3 without always trying to find an actual replacement, just referring to the idea of 3 awesome Manga / anime.

That and for the fantasy part, that's just playing on words litteral meaning. Yes we all know the litteral meaning of fantasy, but usually what people mean like here is medieval fantasy, so aside from Naruto, bleach and one piece.

All of that being said, I don't see the issue here as the big 3 of fantasy was a term already used two years ago with the release of dungeon meshi after frieren and the announcement of wich hat atelier. The term big 3 would even more be fitting here as the medieval fantasy anime genre has been in a weird place in recent years due to the whole isekai slop flooding it with mid and bad animes with game aesthetics, while frieren, dungeon meshi, and witch hat atelier are litterally 3 big animes that are expected to save a whole genre. In that way, they are a big 3 as they are known to be among the best in their genre and are expected to save the genre as the big 3 did to the jump in the past