Wrap every 3 elements with div.row:

$index = 1;

foreach( $sub_pages as $sub_page ) {
	
	if ($index % 3 == 1 || $index == 1) { // beginning of the row or first
		echo '<div class="row">';
	}
	
	echo '<div class="col-sm-4">';

	echo $sub_page->post_title;
	echo $sub_page->post_excerpt;
	
	echo '</div><!-- .col -->';
	
	if ($index % 3 == 0 || $index == count($sub_pages)) { // end of the row or last
		echo '</div><!-- .row -->';
	}
	
	$index++;
}

Wrap every 5 elements using array_chunk():

$chunks = array_chunk($original, 5);
foreach ($chunks as $each_chunk) {
        // echo out as unordered list
}