The Black Mamba theme is based on the Waffles which in turn is based on the Acquia Marina 2.x theme. Going all the way back to the source the Acquia Marina 2.x has a feature where the third column is actually inline or nested inside the second column. This is a feature the authors of Acquia Marina 2.x added, but has caused issues for many people including me. I just figured out how to change this theme so it's 3 standalone columns with no inline stuff. This should also work with the Waffles theme and probably even the Acquia Marina 2.x theme.

Open up page.tpl.php in your Black Mamba folder. Locate the code for the right column

              <?php if (($sidebar_first) && ($sidebar_last)) : ?>
                <?php if ($sidebar_last): ?>
                <div id="sidebar-last">
                  <?php print $sidebar_last; ?>
                </div><!-- /sidebar_last -->
                <?php endif; ?>
              <?php endif; ?>

Select and cut the code out. Then scroll up to where the code is for the left column. It should look like this.

          <?php if ($sidebar_first): ?>
          <div id="sidebar-first">
            <?php print $sidebar_first; ?>
          </div><!-- /sidebar-first -->
          <?php endif; ?>

Just below that code insert the code you previously cut. And just to be crystal clear, you aren't deleting any lines of code, all you are doing is moving 7 lines of code for sidebar-last up so it's just under sidebar-first. That's it, you should now have a full 3 column theme without any columns being nested inside each other. You will most likely have to edit your style.css and mess with the margins or possibly padding for #sidebar-last so it's lines up like sidebar-first