WordPressの投稿ループでBoostrapを使い3列表示するサンプルです。

wp-loop-col-lg-4.php

<?php
global $posts;
$num = 1;
?>
<?php foreach ( $posts as $post ) : setup_postdata( $post ); ?>
<?php if ( 1 === $num || 1 === ( $num % 3 ) ) : ?>
	<div class="row">
<?php endif; ?>
	<div class="col-lg-4">
		<div class="thumb-panel__img">
			<a href="<?php echo esc_url( get_permalink() ); ?>">
				<?php the_post_thumbnail(); ?>
			</a>
		</div>
	</div>
<?php if ( ( 0 === ( $num % 3 ) && 1 !== $num ) || count( $posts ) === $num ) : ?>
	</div>
	<!-- .row -->
<?php endif; ?>
<?php $num ++; ?>
<?php endforeach; ?>