Mary Watson!

[katikat]

She’s a highly trained intelligence agent, who’s doing a lot better job of breaking in than John and Sherlock are. And if you actually think this through, suppose Sherlock hadn’t blundered his way in that night? She’d just have shot Magnussen, gone back to being Mrs. Watson – and not only that, they’d have carried on solving crimes together, with this lethal killer nurse wandering along behind them, picking off anyone who might put them in danger. That would’ve been the show.

Source

Moffat said them, who might put THEM in danger! They really are a fully functioning threesome, aren't they?