Tardis:Rule of three

Categories must generally have three members or sub-categories in order to avoid deletion. Exceptions to this rule are purely technical. They include cases where:
 * The category would obviously contain three members, but the pages simply haven't been written yet. For instance, on the day that Category:Politicians was created, the wiki was very young, and therefore there weren't, in fact, three politicians yet possessing pages.  Another good example is the case of the fairly obscure "from the real world" (FTRW) category.  Instrumental compositions from the real world for instance was once young enough to contain only one entry — but it was fairly obvious at the time of creation that it would ultimately hold more.
 * The category is necessary to link one branch of the category tree to another. As examples, a lot of the sub-categories of Centuries have only one year within them, particularly for those centuries in the far future. But the category is still necessary for the category tree to function logically, and for the  template to work.
 * The category is about crew members and it's necessary to adequately describe a person's job, even though that person may have been one of two or fewer people to ever hold that title.