But it overflow method does work for my test three column page in Chrome and Safari on Windows, but on Firefox and IE7 on Windows, the maincontent now clears the LEFT sidebar instead (it might always have done but the right sidebar is longer), whereas before it was clearing the right sidebar. I also had to manually set the width of the main content as it had narrowed in all browsers to less than 1/2 the auto width for the main content.

This is getting much closer to the solution of the issue of the main content clearfloats forcing the content below the right sidebar. How do I deal with the left one now!? Any comments, anyone, and thank you Vinh Parm!