title: Fast extraction of elements from nested lists
body: <p>This is a basic question on list manipulation in Mathematica.
  I have a large list where each element has the following schematic form: {List1, List2,Number}. For e.g., </p>
  
  <p><code>a = {{{1,2,3},{1,3,2},5},{{1,4,5},{1,0,2},10},{{4,5,3},{8,3,4},15}}}</code>.</p>
  
  <p>I want to make a new lists which only has some parts from each sublist. Eg., pick out the third element from each sublist to give {5,10,15} from the above. Or drop the third element to return {{{1,2,3},{1,3,2}},{{1,4,5},{1,0,2}},{{4,5,3},{8,3,4}}}.</p>
  
  <p>I can do this by using the table command to construct new lists, e.g.,</p>
  
  <pre><code>Table[a[[i]][[3]],{i,1,Length[a]}
  </code></pre>
  
  <p>but I was wondering if there was a must faster way which would work on large lists. </p>