Should I sleep with my ex's girlfriend's ex
It depends. Why do you want to sleep them?
It will get back at them my ex and her girlfriend.
It can be hard to see your ex doing good and wanting to hurt them is a normal feeling. But you cannot act on it and betray their trust because everybody knows that is wrong even if you are not dating.
Nah. I don't care. She will forgive me. I'll just tell my ex that I did it because she left me.
You should really reconsider sleeping with them. You can't count on her forgiving you. Even if she does she'll lose some amount of trust in you.