Iterate through Firebase data randomly?

Everytime the page loads, I want to display the items in my Projects node in a random order, with varying sizes as well.

So, I know how to access one item randomly as so:

var i = 0;
var rand = Math.floor(Math.random() * snapshot.numChildren());
snapshot.forEach(function(snapshot) {
  if (i == rand) {
    // picked random item, snapshot.val().
  }
  i++;
});

but I don't know how to do it multiple times for multiple items. I came up with this:

var numItemsPerLoad = 3;
    for(var i = 0; i < numItemsPerLoad; i++){
        ref.limitToFirst(numItemsPerLoad).on("value", function(snapshot){
            var rand = Math.floor(Math.random() * snapshot.numChildren());
                var j = 0;
                snapshot.forEach(function(snapshot){
                    var li = document.createElement("li");
                    var img = document.createElement("img");
                    var iframe = document.createElement("iframe");
                    if(j == rand){
                        img.setAttribute("src", snapshot.val().url);
                        //These commented ones will randomize the height and width, but it'll look stretched and ugly sometimes
                        //img.setAttribute("width", randomNum(100, 250));
                        //img.setAttribute("height", randomNum(100, 250));
                        li.appendChild(img);
                        ul.appendChild(li);
                    }
                    j++;
            });//end snapshot forEach
    });

But when I do it like this, where I try to randomly choose an item three separate times, it might choose an item it chose before, so it'd display the same item twice or thrice even. Is there a better/more efficient way to randomly display ALL the items in a node?