Probably the best page out there on the 3-day diet.

A good page on the 3 day diet that immediately (paragraph 3) points out that the claim of 10lb lose is fluid lose and not fat loss which is nearly impossible. Any diet that claims large losses in a minimal amount of time and based on you losing water weight which isn't healthy and will only be regained once you are off the diet or on it long enough for your body to adjust. Another bad idea diet!

This site first outlines the strict meal plans of the 3 day diet and then explains why it's not a healthy way to eat. Essentially, dieters are setting themselves up for 3 days of highly restrictive eating (under 1000 cal per day) followed by 4-5 days of overeating to compensate.