title: How to sum up a sequence of value in 3 records span?
body: <p>Sorry for my broken English, I know the title is weird. But I want to do is like below.<br>
  Let' s say I have a <code>Series</code> </p>
  
  <pre><code>pd.Series([1,3,2,1,4,3])
  
  0    1
  1    3
  2    2
  3    1
  4    4
  5    3
  dtype: int64
  </code></pre>
  
  <p>I want to add each record with the former 2 records and get the Series below.<br>
  For instance, for 5th record, the new value 8 = 1 + 4 + 3.</p>
  
  <pre><code>0    1
  1    4
  2    6
  3    6
  4    7
  5    8
  dtype: int64
  </code></pre>