W3Schools Tryit Editor

<!DOCTYPE html>
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<title>W3.CSS</title>
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://www.w3schools.com/w3css/4/w3.css">
<link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/4.7.0/css/font-awesome.min.css">
<body>

<div class="w3-sidebar w3-bar-block w3-light-grey w3-card" style="width:30%">
  <a href="#" class="w3-bar-item w3-button">Link 1</a> 
  <a href="#" class="w3-bar-item w3-button">Link 2</a> 
  <div class="w3-dropdown-hover">
    <button class="w3-button">Dropdown
      <i class="fa fa-caret-down"></i>
    </button>
    <div class="w3-dropdown-content w3-bar-block">
      <a href="#" class="w3-bar-item w3-button">Link</a>
      <a href="#" class="w3-bar-item w3-button">Link</a>
    </div>
  </div> 
  <a href="#" class="w3-bar-item w3-button">Link 3</a> 
</div>

<div class="w3-container">
<h2>Dropdown in Sidebar</h2>
<p>In this example, we have added a dropdown menu inside the sidebar.</p>
<p>Notice the caret-down icon, which we use to indicate that this is a dropdown menu.</p>
</div>

</div>
      
</body>
</html>