Yeah it's kind of tricky to get though.
Eva cannot be bad
Ash should encourage Eva to be naughty
Ash has to be dominant towards Eva and opted for an open relationship,
Jess still has to be bad
Ash has to have inquired about threesomes and cheated on Eva with Jess.
I'm pretty sure that Ash has to also move in with Jess
Ash cannot have had sex with any men as far as I'm aware, could possibly still have sex with Eric on their first date in the game.
After proposing an open relationship with Eva, Ash will then propose a threesome, if you're not having sex with any men, Ash can suggest finding another woman for the threesome. Nat or Iris are the default, if the other requirements are met then Ash can call Jess and convince her. Jess then doms Eva, Eva admits to enjoying it, and then it's never brought up again by any of the three.