Unimplicating threesome -> sex.
Reason: AFAIK the sex tag is only supposed to be used for actual penetrative intercourse. Threesomes don't necessarily involve intercourse, especially yuri threesomes.
Updated by a moderator
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I disagree. The sex tag, as per the wiki as of this moment, also includes cunnilingus, which is not necessarily itself an act of penetrative sex. fingering, too, is on the list, and it's such a mainstay of yuri... well, you see where I'm going with this.
In fact, I would suggest that tribadism should imply sex as well.
I don't think what counts as sex on Danbooru has ever really been discussed. I've always been under the impression that it is strictly for vaginal/anal intercourse and that anything else isn't sex for our purposes. Otherwise I'm pretty sure we would've implicated fellatio, cunnilingus, fingering, etc to sex long ago.