title: Replace any 3 letters from the string with random letters
body: <p>I want to replace any 3 random letters from the string with random letters from my <code>letters</code> variable</p>
  
  <pre><code>var str = "HELLO";
  var letters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
  var arr = str.split('');
  for (var i = 0; i &lt; 3; i++) {
      var pos = Math.round(Math.random() * arr.length - 1);
      arr.splice(Math.floor(Math.random() * arr.length), 1);
      str = arr.join('');
  }
  alert(str);
  </code></pre>
  
  <p>I am able to take 3 random letters out right now but can't figure out how to get 3 random letters from <code>letters</code> and put them in a random position.</p>
  
  <p>Here is the demo of what I have right now.</p>
  
  <p><a href="http://jsfiddle.net/r1zd7rsw/" rel="nofollow">http://jsfiddle.net/r1zd7rsw/</a></p>
  
  <p>Any help would be appreciated!</p>