$form->addRadioList('type')->setItems(array(1, 2, 3));
$form->addText('first');
$form['type']
    ->addConditionOn($form['type'], $form::IS_IN, array(1, 2))
    ->toggle($form['first']->htmlId)

$form->addText('second');
$form['type']
    ->addConditionOn($form['type'], $form::EQUALS, 2)
    ->toggle($form['second']->htmlId)