Depending on if you want this sidebar to be present in all pages or only on one page:

All Pages

In release 0.9 of XWiki you you just need to create a page XWiki.Menu in your Wiki. It will automatically added to all the pages on your Wiki.

In only one page

You can use the following HTML in your page:

<div class="left">
left block 1
</div>
<div class="left">
left block 2
</div>
<div class="center">
main content
</div>

Be carefull not to add white lines between the div blocks since they are transformed into paragraphs by the wiki syntax and Internet Explorer doesn't like it.