Both are correct...
As an abbreviation it applies to both, a collective term for the three things, and for one of them at the same time. It's the result of trying to use Japanese terminology under English-language conventions, but typically most people are only using it to refer to netorare/cuckold stuff.
It's basically because of lazy hentai weebs who spread the use to lazy non-weebs and the desire/need to categorise things under specific collective headings in a manner that appeases both the lazy weebs and non-weebs who've become obsessed with using those terms.
I'm still amused that the old NTR tag was changed to netorare, whilst netorase and netori aren't used as tags, but their English-language equivelants are... It's almost like they're afraid to use the term cuckold as a tag.
