title: Use str.join on strs in columns in df
body: <p>Is there a way to use join here?</p>
  
  <pre><code>df = pd.DataFrame([
      ['a','b','c']
  ])
  df['combined'] = df[0] + ':' + df[1] + ':' + df[2]
  df
  </code></pre>
  
  <p>This is what I have tried:</p>
  
  <pre><code>df['combined'] = ':'.join([df[0], df[1], df[2]])
  </code></pre>