Hi luke984,
You need to open templatesja_argotplsblocksmainbody-content-left.php, find lines (74th):
<!-- SIDEBAR -->
<!-- MAIN CONTENT -->
<div id="t3-content" class="t3-content <?php echo $hasSidebar ? 'span9':'span12' ?>" data-tablet="span12" data-xtablet="span12">
<div class="t3-sidebar span3" data-tablet="span9 pull-right" data-xtablet="span9 pull-right">
Change span3 to span4.
On T3 templates, we use the Bootstrap’s grid system with 12 columns (span) to display the template layout. So that, you will need to change the t3-content width also when you change sidebar’s width (span), change span9 to span8