Showing the post only when there is title, thumbnail and the content in Custom Fields

by

You can use get_the_content to a variable instead, so you can check it is not empty first:

<?php $content = get_the_content(); ?>
<?php if (!empty($content)) : ?>
    <h3><?php the_title(); ?></h3>
    <p><?php echo $content; ?></p>
<?php endif; ?>