Just to make sure we are on the same boat, here is how I understand your situation:
You want to have three types of news items - normal, see (I don't know what that is), and important. You want these news items to be styled differently in the list. You have setup css for three different css classes, and you want to apply different class, depending on the type of news item.
First, I want to clarify that the news items in a single list cannot be styled differently, because in the back end there is no difference between them. If you want to have three separate NewsLists styled differently, then you need to create three templates and manually set the template for each one by using the TemplateUrl property of the NewsList control. Then you can create three categories for news, and bind the different NewsList controls to the respective category.
The role of the templates for the controls is to take out the presentation of the control and separate it from code. You can also provide multiple templates for the same control in different scenarios. However, the templates are not controls by themselves (although they are .ascx files), and cannot be modified in design time or runtime. Your only solution in this case is to create separate templates. Changing css classes by a dropdown in design time cannot be done with a template.
Please excuse me if I misunderstood your problem. In this case, tell us clearly what you want to achieve and we will do our best to help you.
the Telerik team