change your query to:
<?php $my_query = new WP_Query( array("cat" => 3, "posts_per_page" => 4, "post__not_in" => array($mainPost)));
http://codex.wordpress.org/Function_Reference/WP_Query#Post_.26_Page_Parameters
change your query to:
<?php $my_query = new WP_Query( array("cat" => 3, "posts_per_page" => 4, "post__not_in" => array($mainPost)));
http://codex.wordpress.org/Function_Reference/WP_Query#Post_.26_Page_Parameters