public void Show(string _title , string _message, bool _showYesButton, bool _showNoButton, bool _showCancelButton) {
    this.gameObject.SetActive (true);
    Title.text = _title;
    Message.text = _message;
    btnYes.SetActive (_showYesButton);
    btnNo.SetActive (_showNoButton);
    btnCancel.SetActive (_showCancelButton);


}

MyMessageBoxScript _script = MyMsgBoxGameObject.GetComponent<MyMessageBoxScript >();
_script.Show("Hey..","Are you sure you want to overwrite this save ?",true,true,false);

buttonPressed = _script.Show("Hey.:", "Are you sure...");

_script.Show(CallBackFunction; "Hey..","Are you sure you want to overwrite this save ?",true,true,false);

void CallBackFunction () {
...
}