Hi,
The mainnav is not a position actually so don't change it. If you are using T3 BS3 Blank template, you can change the width of the mainnav by opening the file: templates\t3_bs3_blank\tpls\blocks\mainnav.php then define the width for the mainnav with "col-md-". Here is a sample:
<div class="col-md-6">
<!-- Brand and toggle get grouped for better mobile display -->
<div class="navbar-header">
<?php if ($this->getParam('navigation_collapse_enable', 1) && $this->getParam('responsive', 1)) : ?>
<?php $this->addScript(T3_URL.'/js/nav-collapse.js'); ?>
<button type="button" class="navbar-toggle" data-toggle="collapse" data-target=".t3-navbar-collapse">
<i class="fa fa-bars"></i>
</button>
<?php endif ?>
<?php if ($this->getParam('addon_offcanvas_enable')) : ?>
<?php $this->loadBlock ('off-canvas') ?>
<?php endif ?>
</div>
<?php if ($this->getParam('navigation_collapse_enable')) : ?>
<div class="t3-navbar-collapse navbar-collapse collapse"></div>
<?php endif ?>
<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>
</div>
Regards.