小三虽美但终究抵不过原配
[xiăo sān suī mĕi dàn zhōng jiū dĭ bù guò yuán pèi]
The term here means that while a mistress ('小三') may be attractive, she can never surpass or replace the wife ('原配', original/legitimate wife). It points out the idea that no matter how desirable or appealing something or someone might seem as an alternative choice, it will never outweigh the value and place of what's original.