We have the details in our diet guide here
http://www.3fatchicks.com/diets/3-day-diet.php
There are several versions of it going around, but they are all similar, and result in the same thing: water loss.
It helps you drop excess water weight, not fat. It's also not nutritionally sound, so you should never follow it long, if you do it at all. This is one of the oldest fad diets around.
You have to burn 3500 calories to lose just one pound of fat, and you can only safely lose 1 to 2 pounds of fat per week. Anything else to go is usually water or lean muscle.