One of them is reluctant about anal sex. So the second sister says to me, "Do you know where triplets come from? A single egg splits in the womb. That means we're more than just sisters. Biologically, we're literally the same person. So if I say you can have this ass, you have consent because it's my ass." Then the third sister joins in saying, "That's right. Since we're the same person, and I give consent as well, that means two-thirds of me wants you to have sex with my ass."
The reluctant sister protests a little, but it's just for show as we agreed upon a safety word if anything was really off limits and she chooses not to be a spoilsport.