If you are speaking of the option at the top od the editing page saying
“Display page content in this many columns using CSS column rules. “
You don’t have any control. The content will flow in the 3 columns automatically as needed to fill up 3 columns.
This allows you to be sure you never end up with partially filled columns, or with a 3rd column that is longer that the others.
If you want 3 columns with a fixed amount of content, you need to create them yourself with one of the methods described in the thread below
http://forum.weavertheme.com/discussion/11220/weaver-xtreme-tips-and-tutorial-part-5