<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Flexbox</title>
</head>
<body>
<div class="container">
  <div class="item">1</div>
  <div class="item">2</div>
  <div class="item" id="flex">3</div>
</div>
</body>
</html>
 .container{
            display: flex;
            height: 70px;
            border: 1px solid #808080;
           }

.item {
       border: 1px solid #1d1d1d;
       background-color: #23C4ED;
       width: 50px;
       height: 50px;
       margin: 7px;
      }

#flex{
  flex-grow: 3;
}

External CSS

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External JavaScript

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