小三我当不起
[xiăo sān wŏ dāng bù qĭ]
The meaning is 'I cannot be the third party (mistress/cheater)'. This suggests that the user may not be comfortable being involved in someone else's relationship. There is a strong social disapproval of cheating or infidelity in China, and this nickname might express refusal or criticism of such actions from a personal level.