How can I produce an alert if three out of nine checkboxes are checked?

I have nine checkboxes linked to nine images and three of them use the name 'correct' using the code shown below.

<div class="nine">
            <label for="correct1"><img class="picture1" src="picture1.jpg"/></label>
            <input type="checkbox" class="chk" id="correct1" name="correct"/>
            </div>

The remaining six are unnamed using the code shown below.

<div class="nine">
            <label for="incorrect1"><img class="picture4" src="picture4.jpg"/></label>
            <input type="checkbox" class="chk" id="incorrect4"/>
            </div>

I currently have the following code to produce an alert if the three checkboxes with the name "correct" are checked but it isn't working.

<script>
var i, correct = document.getElementsByName('correct');
            for (i = 0; i <= correct.length; i++) {
                if (correct[i].checked) {
                    alert('correct');
                    return true;
                }
            }
            alert('incorrect');
            return false;
            </script>

Can anyone help me with this?