CSS Category Pages – width problem

Hey,
I’ve got the wp amazillionaire theme and did a few configurations in the css. My problem are the category pages. There three Posts should be in each row. Now there are only two post in each row although there is enogh space next to the second post.
Its hard to explain, here is the link: http://www.werkstattwagentest.de/
where can i edit the css to solve this problem?

greets