How do I have a threesome with Yen and Triss?

I've seen the actual scene in which this occurs, but I am not sure how to get it to happen in my game.

I've played through twice and I've either gone with full Yen or full Triss. Is there a point in the story in which I could choose both and they both find out?

Here is the scene in which I am referring to.

To have this option you have to:

1. admit love to Yen during the Quest The Last Wish.
2. admit love to Triss during the Quest Now or Never.

Then you will automatically come into the scene, also look at this video:

Based on the question itself, you cannot have "a threesome" with Yen and Triss.

Based on the text within: You can continue a relationship with both, individually, throughout the game, and the scene shown in the video happens at Kaer Morhen when gathering followers if you've... well, led them both on.