To be honest, a "Harem" ending already exist ...
A harem without freewill is not really a harem. It's a collection of human sized sex toys.
Also, I accomplished the Callie/Evelyn/MC romance path.
So basically:
Get Callie to the point of Naked Fashions shows, and that the "Cum together" option is available.
Get Evelyn's Dark Secret revealed, but don't confess to her.
Suggest that she have fun with Callie at least 5 or 6 times, I think I stopped at 6. Watch, don't join.
Spend enough time with Evelyn that you've gotten to the naked massage stage.
Admit your feelings to Evelyn, have sex with her the first time and at least one additional time after that.
Ask her to go to Callie again, this time join them. Only ask to do stuff with Evelyn.
Ask Callie to have sex the next day. Make sure you don't cum in her like she asks.
By the time the next night rolled around I had a conversation with Evelyn about her talking to Callie about the Triangle Relationship. You can accept or not (seriously, who's not accepting)
Things to note:
This replaces the Picnic/Private sex time with Callie. You'll only get the fully lit bedroom on the threesome scenes now, the private Callie romance scenes are only for the single romance route.