<nav class="navbar navbar-expand-lg navbar-dark bg-primary">
  <a class="navbar-brand" href="#">Mega Dropdown</a>
  <button class="navbar-toggler" type="button" data-toggle="collapse" data-target="#navbarSupportedContent" aria-controls="navbarSupportedContent" aria-expanded="false" aria-label="Toggle navigation">
    <span class="navbar-toggler-icon"></span>
  </button>

  <div class="collapse navbar-collapse" id="navbarSupportedContent">
    <ul class="navbar-nav mr-auto">
      <li class="nav-item">
        <a class="nav-link" href="#">Category</a>
      </li>
      <li class="nav-item dropdown">
        <a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
          Category 1
        </a>
        <div class="dropdown-menu" aria-labelledby="navbarDropdown">

          
          <div class="container">
            <div class="row">
              <div class="col-md-4">
                <span class="text-uppercase text-white">Category 1</span>
                <ul class="nav flex-column">
                <li class="nav-item">
                  <a class="nav-link active" href="#">Active</a>
                </li>
                <li class="nav-item">
                  <a class="nav-link" href="#">Link item</a>
                </li>
                <li class="nav-item">
                  <a class="nav-link" href="#">Link item</a>
                </li>
              </ul>
              </div>
              <!-- /.col-md-4  -->
              <div class="col-md-4">
                <ul class="nav flex-column">
                <li class="nav-item">
                  <a class="nav-link active" href="#">Active</a>
                </li>
                <li class="nav-item">
                  <a class="nav-link" href="#">Link item</a>
                </li>
                <li class="nav-item">
                  <a class="nav-link" href="#">Link item</a>
                </li>
              </ul>
              </div>
              <!-- /.col-md-4  -->
              <div class="col-md-4">
                <a href="">
                  <img src="https://dummyimage.com/200x100/ccc/000&text=image+link" alt="" class="img-fluid">
                </a>
                <p class="text-white">Short image call to action</p>

              </div>
              <!-- /.col-md-4  -->
            </div>
          </div>
          <!--  /.container  -->


        </div>
      </li>
      <li class="nav-item dropdown">
        <a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
          Category 2
        </a>
        <div class="dropdown-menu" aria-labelledby="navbarDropdown">


          <div class="container">
            <div class="row">
              <div class="col-md-4">
                <span class="text-uppercase text-white">Category 2</span>
                <ul class="nav flex-column">
                <li class="nav-item">
                  <a class="nav-link active" href="#">Active</a>
                </li>
                <li class="nav-item">
                  <a class="nav-link" href="#">Link item</a>
                </li>
                <li class="nav-item">
                  <a class="nav-link" href="#">Link item</a>
                </li>
              </ul>
              </div>
              <!-- /.col-md-4  -->
              <div class="col-md-4">
                <ul class="nav flex-column">
                <li class="nav-item">
                  <a class="nav-link active" href="#">Active</a>
                </li>
                <li class="nav-item">
                  <a class="nav-link" href="#">Link item</a>
                </li>
                <li class="nav-item">
                  <a class="nav-link" href="#">Link item</a>
                </li>
              </ul>
              </div>
              <!-- /.col-md-4  -->
              <div class="col-md-4">
                <a href="">
                  <img src="https://dummyimage.com/200x100/ccc/000&text=image+link" alt="" class="img-fluid">
                </a>
                <p class="text-white">Short image call to action</p>

              </div>
              <!-- /.col-md-4  -->
            </div>
          </div>
          <!--  /.container  -->


        </div>
      </li>
      <li class="nav-item dropdown">
        <a class="nav-link dropdown-toggle" href="#" id="navbarDropdown" role="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
          Category 3
        </a>
        <div class="dropdown-menu" aria-labelledby="navbarDropdown">


          <div class="container">
            <div class="row">
              <div class="col-md-4">
               <span class="text-uppercase text-white">Category 3</span>
                <ul class="nav flex-column">
                <li class="nav-item">
                  <a class="nav-link active" href="#">Active</a>
                </li>
                <li class="nav-item">
                  <a class="nav-link" href="#">Link item</a>
                </li>
                <li class="nav-item">
                  <a class="nav-link" href="#">Link item</a>
                </li>
              </ul>
              </div>
              <!-- /.col-md-4  -->
              <div class="col-md-4">
                <ul class="nav flex-column">
                <li class="nav-item">
                  <a class="nav-link active" href="#">Active</a>
                </li>
                <li class="nav-item">
                  <a class="nav-link" href="#">Link item</a>
                </li>
                <li class="nav-item">
                  <a class="nav-link" href="#">Link item</a>
                </li>
              </ul>
              </div>
              <!-- /.col-md-4  -->
              <div class="col-md-4">

                <a href="">
                  <img src="https://dummyimage.com/200x100/ccc/000&text=image+link" alt="" class="img-fluid">
                </a>
                <p class="text-white">Short image call to action</p>
                
              </div>
              <!-- /.col-md-4  -->
            </div>
          </div>
          <!--  /.container  -->


        </div>
      </li>

    </ul>
    <form class="form-inline my-2 my-lg-0">
      <input class="form-control mr-sm-2" type="search" placeholder="Search" aria-label="Search">
      <button class="btn btn-outline-light my-2 my-sm-0" type="submit">Search</button>
    </form>
  </div>


</nav>
/* adds some margin below the link sets  */
.navbar .dropdown-menu div[class*="col"] {
   margin-bottom:1rem;
}

.navbar .dropdown-menu {
  border:none;
  background-color:#0060c8!important;
}

/* breakpoint and up - mega dropdown styles */
@media screen and (min-width: 992px) {
  
  /* remove the padding from the navbar so the dropdown hover state is not broken */
.navbar {
  padding-top:0px;
  padding-bottom:0px;
}

/* remove the padding from the nav-item and add some margin to give some breathing room on hovers */
.navbar .nav-item {
  padding:.5rem .5rem;
  margin:0 .25rem;
}

/* makes the dropdown full width  */
.navbar .dropdown {position:static;}

.navbar .dropdown-menu {
  width:100%;
  left:0;
  right:0;
/*  height of nav-item  */
  top:45px;
  
  display:block;
  visibility: hidden;
  opacity: 0;
  transition: visibility 0s, opacity 0.3s linear;
  
}
  
 

  
  /* shows the dropdown menu on hover */
.navbar .dropdown:hover .dropdown-menu, .navbar .dropdown .dropdown-menu:hover {
  display:block;
  visibility: visible;
  opacity: 1;
  transition: visibility 0s, opacity 0.3s linear;
}
  
  .navbar .dropdown-menu {
    border: 1px solid rgba(0,0,0,.15);
    background-color: #fff;
  }

}

$(document).ready(function() {
 // executes when HTML-Document is loaded and DOM is ready

// breakpoint and up  
$(window).resize(function(){
	if ($(window).width() >= 980){	

      // when you hover a toggle show its dropdown menu
      $(".navbar .dropdown-toggle").hover(function () {
         $(this).parent().toggleClass("show");
         $(this).parent().find(".dropdown-menu").toggleClass("show"); 
       });

        // hide the menu when the mouse leaves the dropdown
      $( ".navbar .dropdown-menu" ).mouseleave(function() {
        $(this).removeClass("show");  
      });
  
		// do something here
	}	
});  
  
  

// document ready  
});

External CSS

  1. https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/4.1.3/css/bootstrap.min.css

External JavaScript

  1. https://code.jquery.com/jquery-3.2.1.slim.min.js
  2. https://jacoblett.github.io/bootstrap4-latest/bootstrap-4-latest.min.js