TwentyTen > Change Category/Archive ONLY to 3 column layout?

Hi! This is probably a real quicky for someone in the know! I want to change my category/archive view to 3 content columns (4 if you include the side bar) so that the main posts using loop.php look as they do on the main page of My Test Site but, in category, or archive view, they use loop-category.php and I want them layed out as they are on this test page but showing 3 side by side.

HERE is the snippet of loop-category.php that controls how the post should look.

HERE is the code from my category.php page!

Any help would be VERY much appreciated ??