Can I ask why this threesome is considered too rare for a tag that I will immediately use, but there's a tag for, for example, "pairing: jw/sh/irene/mary" which has never been used in a rec and has even only been used on AO3 four times?

And looking to the future, I'm also planning on reccing fics with Sherlock/Gregson, Joan/Marcus and Joan/Ms. Hudson as pairings. . . are those pairings too rare for a tag? And will the Joan/Moriarty fic I'm going to rec go under the existing "pairing: jw/moriarty" tag, or does it need a "pairing: joan watson/moriarty" tag?