title: python pandas how to get multiple columns of grouped result
body: <p>I have a pandas dataframe(named df) as follows:</p>
  
  <pre><code>id, a,  b,  c
  1, 10, 10, 10
  1, 20, 20, 20
  2, 10, 10, 10
  2, 20, 20, 20
  3, 10, 10, 10
  3, 20, 20, 20
  </code></pre>
  
  <p>I need to get result using multiple columns within each group.</p>
  
  <pre><code>grouped = df.groupby('id')
  grouped['a','b','c'].apply(lambda x,y,z:x*y+z)
  </code></pre>
  
  <p>But, the second line has error: </p>
  
  <pre><code>KeyError: ('a', 'b', 'c').
  </code></pre>
  
  <p>How to get this?</p>