Here -suburb of New Orleans-most drivers swing fairly wide-and pass with 5-6 foot margins.
However, it is a rare day that I don't get a 12" pass.
How would cops enforce a 3 foot law? How would they measure a pass?
They would need some sort of gadget to actually prove that the pass was less than 3 feet. A picture, or a video -in many cases- wouldn't be conclusive.
The cops would need some sort of automated gadget to take a picture, and then attempt to make a measurement from that picture or video.
Bike lanes are much easier to enforce.