You don't need clearfix. You need CSS basics 101.
You have a main container - I don't know what its name is, find it. It has probably a fixed width, let's say 700px (not a real number!).
Now inside it you have three (3) divs: your content, sidebar1 and sidebar2. They all have a certain width.
Remember, the width of content+sidebar1+sidebar2 must be less than 700, otherwise they don't fit it in... and will be pushed down. Things will get even more complicated when viewed in IE, where to have to count the borders, margins, padding, too!
Go, and get a 3 column theme that already has what you want. Why to re-invent the wheel?
Since you are in love with Kubrick - why not google for 3 column K? (somebody did today in the forum on my advice and got his problem solved in 5 minutes)