After creating your new article category in Joomla 3.0, you can then assign articles to that category.
This process can be used when creating new articles, or moving an article from one category to another.
The Article Manager Menu
Once inside your admin control panel use the top menu drop-down list click Content and then Article Manager or use the Right side Icon menu Article Manager link to take you to your articles within you website.
Choose Your article Screen
Locate the article you wish to move to a different category.
Then you can either tick the checkbox to the left of the article title and the click the Edit link in the top options menu, or the simplest method is to just click on the article title itself to take you into that article.
Change The Category
Once inside your article, to the right of the title input box you will see the category drop-down list.
Use the dropdown to find the category you wish to move the article to and select it.
Then just click the Save & Close button at the top left of your screen and the article will be saved within that category.