That always reeks of so much insecurity to me. Just own up to it. If you are in a relationship, “sure, I would do a threesome, but only if you actively want it too and we both agreed on the person and what was allowed or forbidden”. Not in a relationship, “Sure, I would be down for a threesome as long as the desires and limits of all people involved are discussed before hand and accepted by all”. There, with that attitude there’s less risk of confusion or misunderstanding.