Wordpress_Related_Articles_Plugin.php

<?php
/**
 * Plugin Name: Show Related Articles
 * Plugin URI: http://esgy.eu
 * Description: List of Related articles at the end of posts
 * Version: 1.0
 * Author: Sorin Gitlan
 * Author URI: http://esgy.eu
 */

add_filter( 'the_content', function($content){

    $id = get_the_id();
    if( !is_singular('post') ){
        return $content;
    } 

    $terms = get_the_terms( $id, 'category' );
    $cats = array();

    foreach($terms as $term){
        $cats[] = $term->cat_ID;
    }

    $loop = new WP_Query(
        array(
            'post_per_page' => 3,
            'category__in' => $cats,
            'orderby' => 'rand',
            'post__not_in' => array($id)
        )
    );

    if($loop->have_posts()){
        $content .= '
            <h2>You may also like...</h2>
            <ul class="related-category-posts">
        ';

        while ($loop->have_posts()) {
            $loop->the_post();

            $content .='
                <li><a href="'. get_permalink() .'">'. get_the_title() .'</a></li>
            ';
            
        }

        $content .= '</ul>';

        wp_reset_query();
    }

    return $content;

});

I am not sure why but it shows 5 Posts :(