Ok, I found I simple solution for the problem:
<?php
$args = array(
'category_name' => 'kampagne-2020',
'post_type' => 'post',
'posts_per_page' => 3,
'orderby' => 'rand'
);
$custom_query = new WP_Query($args);
if ($custom_query->have_posts()) : while($custom_query->have_posts()) : $custom_query->the_post(); ?>
