I am developing a WordPress theme and want to determine if I am at the last post of the current loop page.

<?php get_header(); ?>
    <main id="main">
      <!-- =======  Blog Grid ======= -->
      <section class="news-grid grid">
      <div class="container">
    <?php if (have_posts()) : ?> 
    <?php $counter= 0 ?>
    <?php  while (have_posts()) : the_post(); ?>
            <?php if ($counter==0): ?>
              <div class="row">
            <?php endif; ?>
            <?php $counter++; ?>
                  <?php the_content(); ?>
          <?php if ($counter==3 || get_previous_post() == ""  /* or its the last post this loop page */  ): ?>
            </div>  <!-- close row -->
            <?php $counter=0; ?>
          <?php endif; ?>
     <?php  endwhile; endif; ?>
          </div>
        </section><!-- End Blog Grid-->
    </main><!-- End #main -->
     <?php
    get_footer(); ?>