While loop with an exception after a count is reached

I'm trying to edit the tag.php page so that it displays all tagged posts (as it normally does) 'but' I want to call a function after the 3rd post in the loop.

Current Code

<?php /* The loop */ ?>
<?php while ( have_posts() ) : the_post(); ?>
    <?php get_template_part( 'content', get_post_format() ); ?>
<?php endwhile; ?>

Any suggestions on how to insert the following function after the 3rd post in the loop?

<?php echo someFunction() ?>