jQuery update data attr for element

So I have list of items and I need to add "data-index" for each element, index for first three elements have to be 3, for next 3 elements index have to be 6 …

Should be something like this

<div class="items">
    <div class="item" data-index="3"></div>
    <div class="item" data-index="3"></div>
    <div class="item" data-index="3"></div>

    <div class="item" data-index="6"></div>
    <div class="item" data-index="6"></div>
    <div class="item" data-index="6"></div>

    <div class="item" data-index="9"></div>
    <div class="item" data-index="9"></div>
    <div class="item" data-index="9"></div>
</div>

Anyone have idea how to do it ?

I have this code which add numbers 0, 1, 2, 3, 4, 5…

$('.item').each(function(i) {
  $(this).attr('data-index',e)
})

>Solution :

Try this :

$('.item').each(function(i) {
  var index = 3 * Math.floor(i / 3) + 3;
  $(this).attr('data-index', index);
});