For some context: I'm designing a monster that has 3 heads, which are respectively a Devil, a Yugoloth, and a Demon, fused together into a single body and forced to work together to survive. Preferably, these 3 would have a natural synergy due to having aligning goals.
Zariel was a former Celestial who despised the upper planes' inaction when it came to stopping the Blood Wars, and tried desperately to make the Celestials put their foot down, but after she fell under Baator's control after a brutal loss, she became a Devil.
Rule-Of-Three is a Cambion who works with Graz'zt, and believes that if the three Evil planes came together, they would be able to topple the upper planes, so they started plotting ways to stop the Blood Wars.
Ideally, this Yugoloth character would also want to stop the Blood Wars for some reason, but if there is some other reason they would work with Zariel and Rule-Of-Three, that would work too. It doesn't need to be an especially good reason, just enough to make them work together to survive.
Also, this monster I'm making is directly inspired by Geryon from ULTRAKILL, in fact, it literally is just Geryon. I know that there is already an important Devil named Geryon, but in-universe this creature was named Geryon to confuse people.