For http://www.simplecoding.org/wordpress-vybor-sluchajnyx-postov.html#comment-791883151 comment

Get random posts

<?php
$categories = get_the_category($post->ID);
if ($categories) {
    $category_ids = array();
    foreach($categories as $individual_category) $category_ids[] = $individual_category->term_id;
    $args=array(
        'category__in' => $category_ids,
        'post__not_in' => array($post->ID),
        'showposts'=>-1, // Number of related posts that will be shown.
        'fields' => 'ids',
        'caller_get_posts'=>1
    );
    $posts_ids = new wp_query($args);
    $rnd_ids = array_slice(array_rand($posts_ids), 0 , 3);
    
    // Rest is the same as the previous code
    $my_query = new wp_query(array(
        'post__in'=>$rnd_ids
    ));
    if( $my_query->have_posts() ) {
        echo '<b>Related Posts:</b><p>';
        while ($my_query->have_posts()) {
            $my_query->the_post();
?>
<a href="<?php the_permalink() ?>" rel="bookmark" title="<?php the_title(); ?>"><?php the_title(); ?></a><br />
<?php
        }
        echo '</p>';
    }
    wp_reset_query();
}
?>