title: Using filter with count
body: <p>I'm trying to filter row using the <code>count()</code> helper.
  What I would like as output are all the rows where the <code>map %&gt;% count(StudentID) = 3</code>.</p>
  
  <p>For instance in the df below, it should take out all the rows with StudentID 10016 and 10020 as they are only 2 instances of these and I want 3.  </p>
  
  <pre><code>StudentID StudentGender Grade     TermName      ScaleName TestRITScore
  100             M     9    Fall 2010    Language Usage          217
  100             M    10    2011-2012    Language Usage          220
  100             M     9    Fall 2010    Reading                 210
  10016           M     6    Fall 2010    Language Usage          217
  10016           M     6    Fall 2010    Mathematics             210
  10020           F     7    Fall 2010    Language Usage          210
  10020           F     7    Fall 2010    Mathematics             213
  10022           F     8    Fall 2010    Language Usage          232
  10022           F     9    2011-2012    Language Usage          240
  10022           F     8    Fall 2010    Mathematics             242
  </code></pre>
  
  <p>if I do:</p>
  
  <pre><code>count(df, StudentID)
  </code></pre>
  
  <p>then it only gives me a df with 2 columns, but I want to keep all the columns of my df.  thats's why I think I should use filter.</p>