Three
There are times, Sherlock reckons, where they probably get some really weird looks. Not that he and John hadn't gotten weird looks in the past; yes, Sherlock noticed them, but didn't care enough to take real note. But, the three of them, John, Mary, and himself... well, sometimes, they'd probably get some weird looks.
Like when the three of them go running down some deserted back alley, chasing a guy double their size width-wise, and John has a pistol where Sherlock has throwing stars and Mary's in a formal black dress sans high heels because a date's been interrupted.
Or the times where they're out to dinner and Angelo puts a candle at their table because it's more romantic, and Sherlock's on John's one side and Mary's on the other, and maybe Sherlock does steal pieces of food off of John's plate when John's too busy with Mary but that's hardly his fault because he hasn't had time to eat because of a case.
Or when Sherlock had let himself into the Watson's flat only to find the two occupants on the couch together, naked, doing things, and all Sherlock could do was stare because he honestly hadn't known what they were up to and of course he knew that they had sex but he'd never seen two people so ridiculously in love with each other to the point it made his own ears burn until John yelled at him and Mary just laughed into his neck as Sherlock apologised and let himself back out.
Like the occasions where Sherlock ends up fast asleep in John and Mary's bed, curled up at the foot while John and Mary sleep soundly under the blankets, too, because they've all stayed up so late talking and theorising that they end up, inevitably, sleeping together. (John always complains about it in the morning, but Sherlock always thinks he sleeps well there, himself.)
They probably get weird looks when John is holding Mary's hand and Mary grabs Sherlock's hand and drags him away from one of their suspects and Sherlock lets her, trailing being them in a sulk, because as much as he wants information, he won't kill himself to get even the most stubborn of suspects (not that Mary would let him, not that John would either), and he knows his limits and so does John and Mary.
Or when someone insults Mary to her face and Sherlock's head whips up so fast that it nearly hurts and he rattles off on a deduction about how the insulter's lover is cheating on them and how their child was out of wedlock, laying every ugly detail bare even before John can get to his feet to defend Mary's honour himself (but then John claps him on the shoulder after the whole thing as a silent thank you of appreciation).
Sherlock recognises that people probably think they're a threesome. That's probably why John gets a little bit irritated; although, to be fair, John has stopped being so defensive about the rumours ever since the wedding. Maybe he just doesn't care anymore. Sherlock certainly doesn't. He never has, honestly.
He doesn't notice if they get weird looks, but he figures they probably do. He just knows that he's perfectly content, right where he's at, generally at John's other side but sometimes in the middle if need be. The fact that they probably get weird looks just makes him laugh to himself, when he thinks about it, and mention something to John about the social stigma about three people being romantically involved. John tells him to shut up, of course, and Sherlock laughs again but does, going back to whatever he happens to be working on at the time.
Yes, he's content. He is where he belongs, not a singularity now but now a trio. It's something he never thought he'd be happy about. He never thought that he'd be happy with one friend, but now he has two.
He wouldn't change it for anything. It's a whim of fancy, but he hopes he never has to.
I love Sherlock and John and Mary's relationship... or, at least, their relationship if Mary doesn't die or if the baby doesn't die or if John and Mary get back to the honeymoon stage of their relationship again. I want trio cuddles ;~;
I do not own Sherlock. Thanks for reading!