The funny thing is it didn't even take all that much work for my particular relationship XD My boyfriend had been interested in my girlfriend before he started dating me, but she was seeing someone else at the time and not interested in seeing anyone outside of that. Then a while after that relationship broke up and he realised he was still interested, he asked me if I'd be okay with it. She and I were actually housemates so I knew I got on well enough with her to spend that sort of intensive time together, so all it basically changed was that we went on a lot more actual three-way dates than just "D and K and P hanging out together" not-dates and that they started having sex. I don't sleep with anyone because of a host of reasons, but they're mental and physical rather than not being attracted to them. ...and that is my life story >_>

Mind you, we're a bit of an unconventional relationship even by poly standards, but it's really not that difficult to lay out what everyone is looking for out of a relationship and work out what's compatible with what.