What is the "3 foot law" and which US states have it? Some states seem to have something called the 3 foot law to keep cars away from cyclists.  Why 3 feet?  Which states in the USA have it?
The 3 foot law is an example of a law that exists mostly to create awareness rather than having some direct practical function. 

This law is rarely enforced by itself. Can anyone cite an instance where a motorist was ticketed for passing in under three feet (and not ticketed for anything else)? The 3-foot law is, AFAIK, typically enforced as a supplement to some other violation such as failure to yield or reckless driving.

But more than anything else, the intent is to create some level of awareness in motorists about how to deal with cyclists. If motorists know it is a law, most will make an effort to abide by the law. 

A quick google search reveals that, as of this year, 20 states in the USA have this law.  You can view a map depicting the status of this law in the United States here: http://www.3feetplease.com/important-stuff

It seems to me that in the last 10 years or so, motorists have been getting much better than in the past about getting along with bikes. It could be the marked lanes/signage, or perhaps things like the 3-foot law, but I have noticed a LOT LESS "entitlement-to-the-road" behavior from motorists than in the 1990's and earlier.