3 items requires some more editing of the loop, as currently each row wraps articles two-by-two in the list. For that you would need to modify content.php, archive.php and search.php where it says:
<?php $i = 1; echo '<div class="post-row">'; while ( have_posts() ): the_post(); ?>
<?php get_template_part('content'); ?>
<?php if($i % 2 == 0) { echo '</div><div class="post-row">'; } $i++; endwhile; echo '</div>'; ?>
If you wish to do 4 articles per row as soon as the screen resolution is bigger than 1400px as an example – that is much easier, you would do like this:
@media only screen and (min-width: 1400px) {
.post-list .post-row { width: 50%; border-bottom: 0; }
}