Get random items from firebase min 3 max 5

I am trying to get random items from my firebase database and it almost works, sometimes I only get one or two items when the minimum is 3 and max is 5. Every item has an index and it's getting the items that startAt(startindex) and endAt(startIndex+randomNumber).

Is there any other way to do this other than using an index on each item in the database?

Here is how the code looks now:

    randomNumber = getRandomInt(3,5);
    listView = (ListView)view.findViewById(R.id.listView);
    int startIndex = (int)(Math.random() * childs+1);

    adapter = new FirebaseListAdapter<TraningData>(getActivity(),TraningData.class,R.layout.layout_traning_item,ref_1
    .orderByChild("index")
    .startAt(startIndex)
    .endAt(startIndex+randomNumber)) {
        @Override
        protected void populateView(View view, final TraningData td, int i) {
            //Populating over here!
        }
    };
    listView.setAdapter(adapter);