Hi Bun,
To add the position as you expect, please open the "templates\t3_bs3_blank\tpls\blocks\mainbody\two-sidebar.php" then loads a position inside the Main Content block.
<jdoc:include type="modules" name="<?php $this->_p('position-3') ?>" " style="T3Xhtml" />
Here is an example to add "position-3" to the main content block.
<div id="t3-mainbody" class="container t3-mainbody">
<div class="row">
<!-- MAIN CONTENT -->
<div id="t3-content" class="t3-content col-xs-12 col-md-6 col-md-push-3">
<jdoc:include type="modules" name="<?php $this->_p('position-3') ?>" " style="T3Xhtml" />
<?php if($this->hasMessage()) : ?>
<jdoc:include type="message" />
<?php endif ?>
<jdoc:include type="component" />
</div>
<!-- //MAIN CONTENT -->
<!-- SIDEBAR 1 -->
<div class="t3-sidebar t3-sidebar-1 col-xs-6 col-md-3 col-md-pull-6 <?php $this->_c($vars['sidebar1']) ?>">
<jdoc:include type="modules" name="<?php $this->_p($vars['sidebar1']) ?>" style="T3Xhtml" />
</div>
<!-- //SIDEBAR 1 -->
<!-- SIDEBAR 2 -->
<div class="t3-sidebar t3-sidebar-2 col-xs-6 col-md-3 <?php $this->_c($vars['sidebar2']) ?>">
<jdoc:include type="modules" name="<?php $this->_p($vars['sidebar2']) ?>" style="T3Xhtml" />
</div>
<!-- //SIDEBAR 2 -->
</div>
</div>
And here is the layout after adding new position
http://easycaptures.com/fs/uploaded/916/9560443538.png
Gardner.