To add sidebar in your phpbb3 template you have to edit two files, overall_header.html and overall_footer.html. These files are in the /forum_root/styles/template_name/template/ directory, where forum_root is the directory where you install your PHPBB forum and template_name is the name of template you want to edit. Default template is prosilver.

<table id="mainbody"><tr><td id="left_sidebar"> <!-- Your sidebar html code here--> </td><td id="right_column">3. Add the following style inside the head tag.
<style>
<!--
table#mainbody{ width:100%; }
td#left_sidebar{width:200px;}
//Add style for your sidebar code.
-->
</style>
4. Open file overall_footer.html for editing.</td> <td id="right_sidebar"> <!-- Your sidebar html code here--> </td></tr></table>4. Open file overall_header.html for editing.
<table id="mainbody"><tr><td id="forum_body">
//Add the following style inside the head tag.
<style>
<!--
table#mainbody{ width:100%; }
td#left_column{width:200px;}
-->
</style>
<style>
<!--
table#mainbody{ width:100%; }
td#left_sidebar{width:200px; vertical-align:top;}
//Add style for your sidebar code.
-->
</style>
<style>
<!--
table#mainbody{ width:100%; }
td#left_sidebar{width:200px;}
//Add style for your sidebar code.
-->
</style>
<style>
<!--
table#mainbody{ width:100%; }
#mainbody td{vertical-align:top;}
td#left_sidebar{width:160px;}
td#right_sidebar{width:160px;}
//Add style for your sidebar code.
-->
</style>3. At the end of that file add the following lines.<table id="mainbody"><tr><td id="left_sidebar"> <!-- Your sidebar html code here--> </td><td id="forum_body">4. Open file overall_footer.html for editing.
</td><td id=?right_sidebar?> //Your right sidebar code here. </td> </tr></table>