Selecting posts older than the current Unix epoch timestamp

This piece of code should solve your problem:

function filter_where($where="") {
    $where .= " AND post_date < '" . date('Y-m-d') . "'";
    return $where;
}

add_filter('posts_where', 'filter_where');
$query = new WP_Query(array('posts_per_page' => 3));
remove_filter('posts_where', 'filter_where');