add a wide sidebar under the 2 right sidebars

Hi !

I’m working on this theme off-line and I use the 3 columns right template (2 sidebars on the right of the page)

The sidebars are each 200 px width (so 400 in all)

I’d like to add a third sidebar that would be 400px width and that would be positioned just under (or above) the 2 sidebars

I followed the instructions here : http://www.deepintheweb.com/tuto-creer-une-zone-de-widget-sur-wordpress/ but I don’t manage to make the sidebar appear at the right place

Could someone help me please ?

Thanks a lot !

Best Regards