Jim always flirts with Sherlock and Mycroft. One day, they take him up on his offer.
The Holmes brothers decide to unify for once in order to dominate Moriarty. Mycroft tells himself he's getting involved because if they play this right, it might get through to Jim enough to dissuade him from his latest mayhem and warn him off messing with Sherlock (whether or not Sherlock believes him on that is questionable). Sherlock gets involved because he finds Jim frustrating, fascinating, and more than a little addictive. They work together in order to dominate Moriarty and make him submit to them.
Mycroft/Jim/Sherlock
The Holmes brothers decide to unify for once in order to dominate Moriarty. Mycroft tells himself he's getting involved because if they play this right, it might get through to Jim enough to dissuade him from his latest mayhem and warn him off messing with Sherlock (whether or not Sherlock believes him on that is questionable). Sherlock gets involved because he finds Jim frustrating, fascinating, and more than a little addictive. They work together in order to dominate Moriarty and make him submit to them.
Of course, Jim being Jim; he loves it.