Graves and Grindelwald are soulmates, but neither really wants to be bonded to the other. After the film, Graves is rescued and takes some time off. Why not visit Scotland? So, he goes there and ends up meeting Dumbledore. The two start seeing each other and decide to have sex. The morning after, Graves wakes up to find Grindelwald and Albus glaring at each other. This leads to a threesome with Graves in the middle.

Bonus if Grindelwald is jealous of both of them.
Double Bonus if Graves has lots of angst over the fact that his soulmate and his boyfriend are clearly into each other.

Triple bonus if Grindelwald and Albus treat Graves differently. Maybe one is gentle and apologetic about the situation, while the other is rough and demeaning. And Graves is ashamed to enjoy being treated both ways