The Confirm window cannot have three button states since the WindowClosedEventArgs exposes DialogResult property which doesn't have three states. In future, we may consider extending our modal dialogs to support more difficult scenarios out of the box, but for now we will keep our current implementation. One of the reason for this is the easiness by which you can define your own modal dialog that resembles the confirm button and introduce new code-behind logic. It is pretty straight-forward how to create a modal dialog and write the Confirm logic yourself.
If you have any other questions or difficulties resolving this one, please don't hesitate to contact us again.
the Telerik team