There is a comprehensive list of towing laws and requirements, including triple-towing, at
http://onlinetowingguide.com/towing_laws.html
The states don't seem to agree on much. For example, many states require that if you tow anything, you carry flares. Everybody got 'em?
Although this site has a lot of info in a compact form, I have trouble getting a scroll bar to show up. Fortunately, there is a PDF version of the table.
Bill