How do I randomly display 3 divs out of a possible 10 in total?

This is what I have tried so far:

HTML:

<div id="1">Content 1</div>
<div id="2">Content 2</div>
<div id="3">Content 3</div>
<div id="4">Content 4</div>
<div id="5">Content 5</div>
<div id="6">Content 6</div>

Javascript:

function randomiseDiv()
{
      // Define how many divs we have
      var divCount = 6;

      // Get our random ID (based on the total above)
      var randomId = Math.floor(Math.random()*divCount+1);

      // Get the div that's been randomly selectted
      var chosenDiv= document.getElementById(randomId);

      // If the content is available on the page
      if (chosenDiv)
      {
            // Update the display
            chosenDiv.style.display = 'block';
      }
}
window.onload = randomiseDiv;

I would prefer a PHP solution, although anything at this stage would be beneficial.