John is an Alpha, happily bonded to Omega!Sherlock.
One day, something goes spectacularly wrong. Either Moriarty drugs John to distract him from protecting Sherlock, and underestimates John's physical strength. It leads to John becoming almost like a mindless beast, escaping his restraints, forcing himself onto Jim, who can be close to his heat, and forcing a bond between them (because, unlike Omegas, some Alphas can bond with more than one partner). Or, somehow, it's Sherlock's fault, though he absolutely didn't mean anything like that to happen! It was just a horrible accident.
As a result, John has got two mates and is horrified when he regains his senses. Jim is his responsibility now, just like Sherlock.
All three are very unhappy about their situation. Jim is their enemy, a criminal mastermind, but John can, in many ways, control him now, even if he doesn't want any of it. He has got two difficult mates to look after and take care of their needs, especially given that neither is very good at taking care of himself. He also has to punish and discipline them when they become too problematic (I prefer no actual BDSM). Sherlock is extremely jealous and distraught, because John was supposed to be only his. He hates sharing. He wasn't into relationships at all before John, but now he has to deal with this. Jim is just as unhappy about all of it. His criminal empire is ruined, he was forced to bond to an Alpha and has lost his freedom and life he had, not to mention that his inner Omega is unhappy, because his mate doesn't love him and doesn't really want him, and only takes care of him, because it's his duty and he's a responsible Alpha who tries to be fair to others.
In other words, it's a nightmare for everyone involved.
It's up to you what happens then. Maybe they try to make it work somehow. Maybe they make a set of rules together to be as comfortable as they can in their difficult and unwanted situation.
John/Sherlock, John/Jim/Sherlock. A/O/O. A!John, O!Sherlock, O!Jim. Rape. Forced Bonding.
One day, something goes spectacularly wrong. Either Moriarty drugs John to distract him from protecting Sherlock, and underestimates John's physical strength. It leads to John becoming almost like a mindless beast, escaping his restraints, forcing himself onto Jim, who can be close to his heat, and forcing a bond between them (because, unlike Omegas, some Alphas can bond with more than one partner). Or, somehow, it's Sherlock's fault, though he absolutely didn't mean anything like that to happen! It was just a horrible accident.
As a result, John has got two mates and is horrified when he regains his senses. Jim is his responsibility now, just like Sherlock.
All three are very unhappy about their situation. Jim is their enemy, a criminal mastermind, but John can, in many ways, control him now, even if he doesn't want any of it. He has got two difficult mates to look after and take care of their needs, especially given that neither is very good at taking care of himself. He also has to punish and discipline them when they become too problematic (I prefer no actual BDSM). Sherlock is extremely jealous and distraught, because John was supposed to be only his. He hates sharing. He wasn't into relationships at all before John, but now he has to deal with this. Jim is just as unhappy about all of it. His criminal empire is ruined, he was forced to bond to an Alpha and has lost his freedom and life he had, not to mention that his inner Omega is unhappy, because his mate doesn't love him and doesn't really want him, and only takes care of him, because it's his duty and he's a responsible Alpha who tries to be fair to others.
In other words, it's a nightmare for everyone involved.
It's up to you what happens then. Maybe they try to make it work somehow. Maybe they make a set of rules together to be as comfortable as they can in their difficult and unwanted situation.