It works!
Here is the code for anyone who's working with Bootstrap and run into same problem. If you have a 4 cols lay-out change
col-md-4 into col-md-3
and
$i % 3 to $i % 4
<div class="container">
<?php
$items = $page->children;
// set the counter to 1
$i = 1;
echo '<div class="row">';
foreach ($items as $item) {
echo "<div class='col-md-4'>
<a href='{$item->url}'><img src='{$item->werk_afbeelding->first()->url}' />
<h4>$item->werk_titel</h4>
<p>$item->werk_intro</p></a></div>";
// After 3 close the row div and open a new one
if($i % 3 == 0) {echo '</div><div class="row">';}
//End count
$i++;
}
?>
</div>
</div>
