It Takes Three to Tango - Novigrad
Location: Novigrad - Kingfisher Inn
Quest Giver: Novigrad Notice Board
Other Requirements: Romance both Yennfer and Triss
During the Final Preparations quest after Ciri has a conversation with Phillipa and Margarita there is a chance that you will be approached by both Triss and Yennefer. If you completed Yennfer's story line and told her you loved her at the end of The Last Wish as well as telling Triss you loved her at the end of Now or Never then you'll get this scene/quest if you agree to their offer when they approach you.
They will imply a three way with you and tell you to go to Kingfisher's Inn with some good wine. Head on over to the Kingfisher's Inn and purchase expensive alcohol if you need to. You may have automatically completed this task if you keep a stock of alcohol on you. Otherwise, 60 Crowns or more will usually update the quest for you. If it doesn't then keep trying to buy things more expensive.
With the alcohol in hand head upstairs to see Triss and Yennefer. All that is left of this quest is a scene between the three of you - not the scene you'd want to see either.