Pasted

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width">
    <title>repl.it</title>
    <style>
      .fleft {float:left}
      .fright {float:right}
      .fclear {clear:left}
    </style>
  </head>
<body>
 
<div style="border:1px solid #cccccc;">
  Qualche testo introduttivo....
 
  <div class="fleft" style="border: 2px solid red;">Scatola 1</div>
  <div class="fright" style="border: 2px solid green;">Scatola 2</div>
  <div class="fleft fclear" style="border: 2px solid red;">Scatola 3</div>
 
  Testo conclusivo.....
</div>
</body>
</html>