I think this is related with theme your using. I mean, if your theme does not provide it out of the box, then you have to edit the sidebar.php of your theme, so it can be devided one column at the top and two column at bottom. Not only that, you have to register additional widget areas for new added blocks. This requires a little bit of PHP programming and WordPress theme structure.
I can’t deep into details, but roughly the layout should be like the following HTML code:
<!-- content for top block goes here -->
<!-- content for bottom left block goes here -->
<!-- content for bottom right block goes here -->