What KitKate said, with one modification:
"Those are the rules I live by. I, personally, won't have a threesome with anynone *in* a relationship...a threesome ruined a friendship and a marriage I know."

The problem with this scenario is not simply involving someone in a prior relationship. The problem comes with that person violating the terms of his/her relationship with his/her SO. Meaning if you do choose to involve such a person, make certain that the SO of that person is also involved in the communication and decision-making process. Which is why:

"Communicate, explore and be safe."

Was so on the nose.

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On the kinkometer, my kink measures as a sine wave.