Nav walker, bootstrap: Display 3rd level items under 2nd level

I solved this problem. follow these instructions…..

1. Add script

   (function($){
   $(document).ready(function(){
       $('ul.dropdown-menu [data-toggle=dropdown]').on('click', function(event) {
           event.preventDefault(); 
           event.stopPropagation(); 
           $(this).parent().siblings().removeClass('open');
           $(this).parent().toggleClass('open');
       });
   }); 
   })(jQuery);
   

2.Remove && $depth === 0
from this line: if ( $args->has_children && $depth === 0 ) in wp_bootstrap_navwalker.php file.

See the screenshot here.