title: Put last 3 entries for each column into a new dataframe when number of columns is variable
body: <p>I have a DF that lists IDs by date like this:</p>
  
  <pre><code>Date          Ben     James
  12/10/17      1294    NA     
  12/11/17      NA      4523      
  12/12/17      8959    3246      
  12/13/17      2345    NA        
  12/14/17      NA      NA        
  12/15/17      0303    8877      
  12/16/17      NA      1427 
  </code></pre>
  
  <p>The number of "name" columns is variable, so on another day I might have a DF that looks like this:</p>
  
  <pre><code>Date          Ben     James     Alex
  12/10/17      1294    NA        3754
  12/11/17      NA      4523      1122
  12/12/17      8959    3246      5582
  12/13/17      2345    NA        NA
  12/14/17      NA      NA        0094
  12/15/17      0303    8877      NA
  12/16/17      NA      1427      NA
  </code></pre>
  
  <p>I want to put the 3 most recent IDs for each name column into a new dataframe, like this:</p>
  
  <pre><code>IDs
  8959
  2345
  0303
  3246
  8877
  1427
  1122
  5582
  0094
  </code></pre>
  
  <p>I just need the IDs in the new DF. I don't care about labeling them by name or date.</p>