Hello chandima_priyantha,
The block that you need modify to add more module position for header is header.php file, not head.php.
So, you need copy this file
<blockquote>plugins/system/jat3/jat3/base-themes/default/blocks/header.php</blockquote>
and paste it into below folder
<blockquote>templates/ja_purity_ii/blocks/</blockquote>
once done, you need add this code snippet for new header.php file:
[PHP]<?php if($this->countModules(‘{module_position}’)) : ?>
<div id=”{module_position}”>
<jdoc:include type=”modules” name=”{module_position}” />
</div>
<?php endif; ?>[/PHP]
replacing {module_position} with name of module position.