some 3-column themes not working

I was using the Seashore theme (3-column, sidebars on right) and all of a sudden the sidebars started appearing at the bottom of the screen below the posts. Nothing I did was changed. I didn’t changed anything on the sidebars. The only thing I did do right before the flub was to post a text-only post.

I looked for other 3-column themes, downloaded about 20. Of these, most had the same error. Examples: Talian, Revolution

A few did work. Examples: Wucoco, Ad Clerum.

I’m a newbie, so I have no idea what’s going on or how to fix it. I really liked the Seashore theme and would like to get it back.

Any help would be greatly appreciated.