U.N.I.T. are a fictional military force that was created to help Doctor 3 defeat threats on Earth. U.N.I.T. are not exclusive to the Third Doctor. They have appeared with Doctor 10 in the long episode Doctor Who - planet of the Dead. U.N.I.T.'s full name is United Nations Intelligence Task force and was originally led by Brigadier Lethbridge-Stewart. They'll return to help the Doctor in future adventures.