I gave my wife three Talaaqs.

Answer

The question of khula' does not develop after you have given three Talaaqs. The Nikah has been irrevocably and finally dissolved. There is now absolutely no scope for reconciliation. She is no longer your wife, hence khula' cannot be made with a woman who is not your wife. Even if you had not issued three Talaaqs, the Mahr cannot be repossessed. The Mahr is the property of the woman.

Assuming that you had not given any Talaaq, then too, khula' cannot be imposed on any of the parties. It is a voluntary agreement between husband and wife. If either does not agree, khula' cannot be contracted.