最接近"the 3rd person"的講法,
可以參考戴安娜在1995年電視訪問中形容她婚姻狀況時所說的:
"Well, there were three of us in this marriage,
so it was a bit crowded."
用"there`s a third person [in our relationship]"來形容這個狀況,也勉強算可以.但是把"3rd person"當做noun使用:"have a third person"就不太通.當然還是可以聽懂想表達的意思,不過並不是正確的講法.
by the way, "the 3rd party"的講法也不是那麼不常用,比較商業或正式的場合還蠻常用到的.