This is pretty straightforward. Let me know if you have any questions with the following code.

var getElementsByClassName = function(className, parent, results) {
  //setup optional parameters
  results = results || [];
  parent = parent || document.body;
  
  //basecase
  if (parent.classList.contains(className))
    results.push(parent);
  
  //recursive case
  for(var i = 0; i < parent.children.length; i++) {
    results.concat(getElementsByClassName(className, parent.children[i], results));
  }
  
  return results;
}

console.log(getElementsByClassName("one"));
console.log(getElementsByClassName("two"));
console.log(getElementsByClassName("three"));

<script src="http://gh-canon.github.io/stack-snippet-console/console.min.js"></script>
<div class="one"></div>
<div class="two"></div>
<div class="one two"></div>
<div class="two three"></div>