Hi,
To fix the issue, please open the file: templates/t3_bs3_blank/tpls/blocks/mainnav.php and find the code:
<div class="t3-navbar navbar-collapse collapse">
<jdoc:include type="<?php echo $this->getParam('navigation_type', 'megamenu') ?>" name="<?php echo $this->getParam('mm_type', 'mainmenu') ?>" />
</div>
Now, replace with the code:
<div class="t3-navbar navbar navbar-collapse collapse">
<jdoc:include type="<?php echo $this->getParam('navigation_type', 'megamenu') ?>" name="<?php echo $this->getParam('mm_type', 'mainmenu') ?>" />
</div>
You may need to add some css to make it look good.
Regards.