Display content after every 3rd post entry in the loop with Genesis

functions.php

<?php

/** pre Genesis 2.0 */
add_action( 'genesis_after_post', 'custom_genesis_after_post' );

/** Genesis 2.0+ */
add_action( 'genesis_after_entry', 'custom_genesis_after_entry' );
/**
 * Callback for Genesis 'genesis_after_entry' action.
 * 
 * Display a box after the entry.
 * 
 * @package Genesis
 * @category Loop
 * @link http://my.studiopress.com/docs/hook-reference/
 * @author Ryan Meier @rfmeier http://www.rfmeier.net/
 *
 * @global int $loop_counter current iteration
 * @return none exit
 */
function custom_genesis_after_entry(){
  
	global $loop_counter;
	
	// if single, return
	if( is_single() )
		return;
	
	// use mode to get the remainder
	$rem = $loop_counter % 3;
	
	// if every 3rd, display the block (example code -- avoid inline styling!)
	if( 0 == $rem )
		echo '<div style="width:100%;height:100px;background:#ff0000;margin-bottom: 40px;">Ad Space</div>';
	
}