I ended up doing this – works perfectly.
<?php $argsb = array('posts_per_page' => -1, 'post_status' => 'publish');?>
<?php $blog = new WP_Query($argsb); if ( $blog->have_posts() ): ?>
<section class="blog-slider slider">
<div class="flexslider">
<ul class="slides">
<?php while ( $blog->have_posts() ) : $blog->the_post(); $i++;?>
<?php if( $blog->current_post % 3 == 0 ) echo "\n".'<li class="slide">'."\n"; ?>
<article class="blog-post-slide">
<?php if ( has_post_thumbnail() ) {?>
<a href="https://wordpress.stackexchange.com/questions/169747/<?php the_permalink(); ?>" title="<?php the_title(); ?>"><div class="blog-slide-image"><?php the_post_thumbnail('blog-slide');?></div></a>
<?php } ?>
<?php the_title();?>
</article>
<?php if( $blog->current_post % 3 == 2 ) echo '</li> <!--/.wrap-->'."\n"; ?>
<?php endwhile; wp_reset_postdata();?>
</ul>
</div>
</section>
<?php endif; ?>