I can’t remember — do you watch BBC’s Sherlock? Because I have a couple of good OT3 fics in that fandom… .
I do, actually - currently going through AO3’s John/Sherlock/Lestrade section. So far they seem to be a bit better about not as much PWP than I usually find. :)
Only problem is, reading them is making me want more poly fic in general. I am such a sucker for it.
If you haven’t found it yet, I can really recommend impishtubist’s Variations on A Theme, which is probably my favorite. Jain’s Beginner’s Studies is fairly good [and entirely about working out the rules of the relationship]. Of Two Detectives and a Doctor is short and fluffy, but it’s definitely not PWP. Also thirdbird’s Triangulations series, which starts John/Lestrade but does work it’s way to OT3.
I’m afraid I don’t have fic for any other poly pairings, but if you haven’t found those you might enjoy them.