It might be likened to “the big three” in some way, but you shouldn’t take it all the way. Many Maamarei Chazal are meant in a certain context and should not be taken out of that. You’ll find yourself with alot of Kaashyos if you do that.
Although Lashon Hara invokes Hashem’s rath and a Lashon Hara group is not Mekabel the Sh’china, It still isn’t one of the “big three”.
As far as Teshuva, as others have written you can do Teshuva on anything. Also, a chronic gossiper would have a hard time recifying his misdeeds, because he can’t trace every person he ever spoke about.