The best solution would be to attach to the OnClientLoad event of the RadRotator and check the length of the items collection. If the length is less than, in your case, 3, then hide the buttons. Hiding the buttons is easily done through the jQuery library. Here is an example implementation of the OnClientLoad handler:
(rotator.get_items().length < 3)
Hope this helps.
All the best,
the Telerik team