title: Python, Add + between every third letter
body: <p>I want be able to add a plus sign (+) for every third letter. 
  For instance,  </p>
  
  <p>sequence = ABCDEF</p>
  
  <p>ABC + DEF</p>
  
  <p>I've tried Str.join and it works sorta, but I want it to add a plus sign every third letter.</p>
  
  <p>This is what I have so far. </p>
  
  <pre><code>s = sequence
  a = (' + '.join(s))
  </code></pre>