$fieldset->addField('title', 'checkboxes', array(
    'label'     => Mage::helper('custommodule')->__('Title'),
    'class'   => 'validate-one-required-by-name',
    'required'  => true,
    'name'      => 'title[]',
    'values' => array(
        array('value'=>'aaa','label'=>'Checkbox1'),
        array('value'=>'bbb','label'=>'Checkbox2'),
        array('value'=>'ccc','label'=>'Checkbox3'),
    ),
    'onclick' => "",
    'onchange' => "",
    'disabled' => false,
    'tabindex' => 1
));