小三你永远是个暖床奴
[xiăo sān nĭ yŏng yuăn shì gè nuăn chuáng nú]
This is a harsh term referring to a mistress or third party in a relationship ('little third') and suggests that they will always just be used for physical warmth and nothing more (like just keeping bed warm). It implies negativity towards the role and future prospect of a mistress.