Specifies whether the checkbox has intermediate state. The default value is false. When the property is enabled the checked() function can return true, false or null.
If you want yo put initially the checkbox into the intermediate state you can set the checked option to null

<input id="checkbox" type="checkbox" ></div>

<script type="text/javascript">
    $(function () {
            label: "checkbox 1",
            enableThreeStates: true,
            checked: null