Last post in loop when even

<?php if( have_posts() ) : ?>

    <?php $count = 0; ?>

    <?php while( have_posts() ) : the_post(); ?>

    <?php
    global $count;

    $col="col-lg-6";

    if($count%3 == 0) {
        $col="col-lg-12";
    } else {
        $col="col-lg-6";
    }

    $count++;

    ?>

    <article id="post-<?php the_ID(); ?>" <?php post_class( $col ); ?>>
        [...]
        [...]
        [...]
    </article>

    <?php endwhile; ?>
<?php endif; ?>