How to add posts via a category in redify theme of wordpress

I am using redify theme of wordpress , i want to know which file of theme i have to edit to place the code to call the post via category. Even dont know which code i have to use for the same. Currently i am using below code but it is not working here.

<?php
                           query_posts('cat=3&showposts=3');
                           while (have_posts()) : the_post();
                               // do whatever you want
                               ?>
      <div class="col-md-4 col-lg-4 text-center">
        <div class="single_promotions text-center">
          <div class="s_promotion_icon"><a href="<?php get_permalink(); ?>"><?php echo get_the_post_thumbnail(); ?></a></div>
          <h2><?php echo get_the_title();?></h2>
          <?php the_content(); ?>
         
        </div>
      </div>
      <?php
                           endwhile;
                           ?>