Once real people are involved, the ethical heart of this whole lifestyle comes down to one thing: the person a couple invites in is a full human being, with her own desires, boundaries, and feelings, not a fantasy made flesh or an accessory to someone else's relationship. Forgetting that is the most common way couples cause harm, and the most avoidable.
See her clearly
The pull of the fantasy quietly encourages treating her as a means to an end. Resist it, consciously and continuously. Seeing her as a whole person changes how you treat her at every stage: caring about her experience and not just your own, respecting her boundaries as seriously as you respect each other's, and never treating her as disposable. The couples who find and keep wonderful connections are, almost without exception, the ones who lead with genuine respect.
Her consent and comfort matter as much as anyone's
Because she is often one person meeting an established couple, there can be an imbalance, two people's wishes against one. Guard against it. Ask about her limits as much as you state yours, give her the same absolute right to pause or stop, and make sure she never feels ganged up on. Use your awareness of that imbalance to protect the person who could be disadvantaged by it, not to exploit it.
Do not discard her
Perhaps the most common and most avoidable cruelty is discarding the third person: wanted intensely for a time, then dropped into silence. However a connection ends, and connections do end, end it with honesty, gratitude, and a real goodbye. How you treat her when you no longer want anything is the truest measure of whether you are doing this well. Treating people with care is both right and, in plain practical terms, what makes them glad to have known you.