I just came here to say that one of my favourite songs has 표현 in the title: 애정표현 by 플라워. :) Enjoy!

The three verbs are VERY different from each other. 표시하다: to mark 표하다: to take a stance 표현하다: to express

표시하다 do make sign. 표하다 showing your decision. 표현하다 do express