Fixing layout of 3 columns

I've a problem with my Theme for wordpress but I got the following effect. I'm trying to modify it (css, html, but until now I've failed)

I've spent already 6 hours without success. What else could I try?

The website is superinformati dot com. I hope this is not seen as spam, since to visit it, you have to copy and edit the text. If someone will see it, means he is motivated to see it.