三儿妹妹莫猖狂
[sān ér mèi mèi mò chāng kuáng]
This username is used to caution or remind a third party (such as a mistress in Chinese context) not to act too arrogantly. '三儿' refers to the mistress of someone, '妹妹' means sister, a term for calling girls friendly or intimate here, and '莫猖狂' means do not be rampant.