<!DOCTYPE html>
<html>
<head>
</head>
<body>
<div style="text-align:center;">
  <button type='button' style="box-shadow:3px 2px">1) My button with a shadow</button>
  <br><br>
  <button type='button' style="box-shadow:3px 2px 3px">2) My button with a blurry shadow</button>
  <br><br>
  <button type='button' style="box-shadow:3px 2px 3px purple">3) My button with a blurry purple shadow</button>
  <br><br>
  <button type='button' style="text-shadow:3px 2px 3px purple">4) My button with a blurry purple shadow on its text</button>
</div>
</body>
</html>


In this modified code, we have wrapped the buttons in a 'div' element and set its 'text-align' property to 'center'. This will center-align all the buttons within the div.
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