상처 : cut or injury that just happened now or a while ago.
ex) 너 손가락에 상처있는데, 언제 다쳤어? You have a cut on your finger, when did you get cut?
흉터 : scar (happened a long time ago)
ex) 이마에 흉터가 있었는데 지금은 사라졌어. I had a scar on my forehead, but it's gone now.
멍자국 : bruise or bruise mark
ex) 무릎에 멍자국 봐봐, 너 문에 부딪혔지? Hey, look at the bruise on your knee, you must've hit a door, haven't you?