Columns in antimatter

I want to make 3 columns setup in the blog page of antimatter theme
in the item twig I have

<div class="pure-u-1-3">

As result I have 2 columns and a large white space. The colums are large exactly 1/3 of the parent column but there is a thin margin between them, so the third item goes to the next line. Here is a screenshot.
Any idea on how to eliminate the white space between the posts blocs?

Is that wrapped in a <div class="pure-g">?