title: get difference between 3 lists
body: <p>I am working on differences of lists.</p>
  
  <pre><code>&gt;&gt;a = [1, 2, 3]
  &gt;&gt;b = [2, 4, 5]
  &gt;&gt;c = [3, 2, 6]
  </code></pre>
  
  <p>Symmetric difference between 2 sets can be done using: </p>
  
  <pre><code>&gt;&gt;z = set(a).symmetric_difference(set(b))
  &gt;&gt;print z
  &gt;&gt;set([1, 3, 4, 5])
  </code></pre>
  
  <p>How to get difference between 3 sets?
  For difference of 3 sets, expected output is : </p>
  
  <pre><code>expected output : set([1, 3, 4, 5, 6])
  </code></pre>