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.fwp-grid {
  display: flex;
  flex-direction: row;
  flex-flow: wrap;
  justify-content: flex-start;
  width: 100%;
}

@media (max-width: 400px) {
  .fwp-grid {
    display: block;
  }
}

.fwp-grid .fwp-grid-col {
  padding: 1em 1em 1em 2em;
  width: 33.3%;
  text-align: center;
}

@media (max-width: 800px) {
  .fwp-grid .fwp-grid-col {
    width: 50%;
  }
}

@media (max-width: 400px) {
  .fwp-grid .fwp-grid-col {
    width: 100%;
  }
}

.fwp-grid .fwp-grid-col h3 {
  padding-top: .25em;
}

.fwp-grid img {
  max-width: 100%;
}

<div class="fwp-grid">
    <?php while ( have_posts() ) : the_post(); ?>
        <div class="fwp-grid-col">
            <?php if ( has_post_thumbnail() ) : ?>
                <a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>"><?php the_post_thumbnail(); ?></a>
            <?php endif; ?>
            <?php the_title( '<h3><a href="' . esc_url( get_the_permalink() ) . '">', '</a></h3>' ); ?>
        </div>
    <?php endwhile; ?>
</div>