How do I format a date value for a 3-column date drop-down widget?
The Form API documentation will help you figure this out, you'll have to pass an array with the year, month and day keys like this:

$form['date'] = array(
    '#title' =&gt; t('Date'),
    '#type' =&gt; 'date',
    '#default_value' =&gt; array('year' =&gt; 2013, 'month' =&gt; 4, 'day' =&gt; 23),
);