① ある事柄や事件に、関係する当事者以外の人。② 特に民事的な法律関係に間接的な利害を有する人。
parent and child meeting (with teacher for course, career guidance, etc.)
each of the three being different from the other two; each of the three having his (her) own way
three-cornered conversation
① 野球やソフトボールなどで、一回の攻撃を三人で終えること。
three-cornered competition
three parties (forces) opposing one another; three-cornered contest