adding is_category to single.php

Hi

I have 3 different post categories that I’m using to filter information on a site that I’m building: project, news and events.

I need to display the single post differently for each type of category with different things in the sidebar. Up until now I’ve been using the single post template plugin which works great but I was wondering whether there’s a way to use a conditional if (is_category(‘n’)) in the single.php somehow so that I don’t have to write a different template for each and more importantly my client doesn’t have to assign a template to their posts each time they write one.