title: using R - delete rows when a value repeated less than 3 times
body: <p>frame with 10 rows and 3 columns</p>
  
  <pre><code>    a   b c
  1   1 201 1
  2   2 202 1
  3   3 203 1
  4   4 204 1
  5   5 205 4
  6   6 206 5
  7   7 207 4
  8   8 208 4
  9   9 209 8
  10 10 210 5
  </code></pre>
  
  <p>I want to delete all rows where the same value in the column "c" repeated less than 3 times.
  In this example I want to remove rows 6, 9 and 10. (my real data.frame has 5000 rows and 25 cols)
  I tried to do it using the function rle, but I keep getting the wrong solution.
  any help? thanks!</p>