How to make the list of post and load on same page with sub-pages [closed]

The page consists of three columns:
1st column: All Parent Pages
2nd column: current page contents
3rd column: Child pages of current page’s parent page

You do it in WordPress by creating a page template like my-custom-page.php within your theme folder and give tour template. This may look like this.

<?php /* Template Name: My Template */ 
   get_header();   ?>
   <div class= "page-contents">
         <div class= "left-col">
            <?php
                // List all top level pages
                $top = wp_list_pages( 'title_li=&child_of=0&echo=0' );
                if ( $top) :
            ?>
                  <ul>
                     <?php echo $top;  ?>
                 </ul>
                <?php endif; ?>
         </div>

         <div class= "middle-col">
         <?php
           if ( have_posts() ) {
              while ( have_posts() ) {
                 the_post(); 
                 //
                 // Post Content here
                 //
              } // end while
           } // end if
          ?>
         </div>

         <div class= "right-col">
            <?php
                // List all sibling pages
                $siblings = wp_list_pages( 'title_li=&child_of=".$post->post_parent."&echo=0' );
                if ( $siblings) :
            ?>
                  <ul>
                     <?php echo $siblings;   ?>
                 </ul>
                <?php endif; ?>

         </div>


   </div> 
  <?php get_footer();  ?>

Once you upload the file to your theme’s folder, go to the Pages > Add New screen in your admin dashboard. here create a new page and assign above created template.

I hope this will help you and give a start and adjust the above code to your needs.

For more information on this these links from WordPress Codex may be helpful:
wp_list_pages()
The Loop