Imagine Person C is a newcomer to Person A and B’s school/university/workplace and they are both instantly attracted. After a lot of flirting and back-and-forth, Person C stops them mid-fight and passionately kisses Person A. As a saddened Person B begins to walk away, Person C pulls them back by their collar and kisses them too. When they are done, Person C smiles, winks and walks away. What happens next is up to you.
Your OTP have an official story about how they met that they tell people whenever they ask, but in reality Person A was working as a waitress at a strip club and Person B was dragged along for a sleazy company meeting. Imagine them discussing this. OT3 bonus: They decide to go back just to see what the place is like now and find an extremely sexy Person C working there.