title: Generate 3 unique numbers from a set of numbers
body: <p>I'm working on a project that makes three spaces of a game board different from the rest (+1,+2,-1) I need to add those values to a variable stored at the index of an ArrayList. 
  I can find something to generate the numbers of the indices, but I don't know how to make sure that they are at unique indices. (It can't put all three of the values in one space)</p>
  
  <p>How do I make three unique, random indices from in the range 0 to 16?</p>
  
  <p>Thanks!</p>