Hello Antonio,
Open your template's index.php and locate the following block of code.
<?php foreach($highlights->entries as $key=>$highlight): ?>
<li class="column large-3 small-6 mobile-4">
The column large-3 small-6 mobile-4 is controlling the width.
If you make into:
<li class="column large-4 small-6 mobile-4">
You will have a 3 columned grid.
