Add a Class to Every nth Post

nthpostclass.php

<?php
/* setup a counter for the sections to use as class */
$counter = 0;

/* loop through each section */
foreach( $sections as $section ) {
	
	/* increment the counter */
	$counter++;
	
	/* start a classes variable */
	$classes = 'section page-section';
	
	/* add classes for section colour depending on the count value */
	if( 1 == $counter % 3 ) {
		$classes .= ' section-white';
	} elseif( 2 == $counter % 3 ) {
		$classes .= ' section-lightgrey';
	} elseif( 0 == $counter % 3 ) {
		$classes .= ' section-midgrey';
	}
	
	/* if this is the first section */
	if( $counter == 1 ) {
		$classes .= ' section-first';
	}
	
	/* if this is the last section */
	if( $counter == $total_sections ) {
		$classes .= ' section-last';
	}
			
	?>
	
	<section class="<?php echo esc_attr( $classes ); ?>">
		
		<!-- // sectoion content here -->
	
	</section>
	
	<?php
	
}
?>