Yes, that is how passing parameters works. If you want to specify a third parameter, for instance, you must also explicitly pass the first and second parameters, regardless of whether they are optional.
First parameter to Form::create is the URI to POST to, default null for current page. Second parameter is an array of attributes--this is where you can set your class.
e.g.
$this->Form->create(null, array('class' => "class1 class2 class3"));