I know several people who have. They then eventually went into P-4. They stabilized very well. One stayed in P-3 for 5 weeks then did 5 weeks of P-4 and then another round.
trudeau encourages you to do only P3 until your final round- but i think that encourages a bit more carb intolerance. but i have no problem with people staying on P3 for longer than 3 weeks. i think too many people look at it as one day they can eat only this (in P3), and the next they can have whatever they want (in P4) which is a dangerous idea. there is nothing magical about those 3 wks, Dr. S. just had to set some type of guideline. i think some people stabilize more quickly than others, depending upon a ton of factors.
i have a friend who has never done a strict P3. but if she has something with carbs she tries to watch her fats. or she backs off for a couple of days after having her "cheat" and eats mostly meat/protein/veggies. it has worked well for her. she's able to "cheat" on the weekends as long as she's more strict during the week.
the main goal of P3 is to keep your weight as steady as possible so your body can "learn" that this is where your set point is supposed to be.
I dunno..this is such a controversial question, it's hard to get a "correct" answer, because, well, just becausethere are many schools of thoughts about it..LOL..;)
I think that Dr. S had you do 3 weeks as a "habit breaking" technique; most "habits" good, bad, or otherwise, are formed and or broken in 21 days. I think he had his patients do 3 weeks just so the "habit" of eating less sugar and starch was really imbedded in the patient. I think after 2 weeks the hcg was definately out of the system, it was just "safer" to say 3 and be sure, and break any "habits". I don't think not eating a carb on the 20th day and the 21st day eating one makes a whole lot of difference. I see a lot of folks on here that do longer than the 3 weeks, but I disagree strongly with that. If their weight is fluctuating so much, I would guess it isn't an hcg issue, but an unknown food reaction/allergen issue. Now that's not to say someone needs a protien day here or there, but I think too many people "overthink" the whole P3/P4 anyway.;)
The video I just watched by Dr Clark says the first three weeks are critical in setting the new fat storage mechanism that we've rested in P2. Anything with starches and fats are deadly to the resetting of it and can really destroy all your work. So Dr. S has us avoid starches altogether to eliminate the problem.
That's the first 3 weeks. But the question was doing it longer than 3 weeks.
I don't think doing it longer makes a hill of beans worth of difference. I could see where Dr. Clark's research would confirm that you do need to do 3 weeks, but it just doesn't seem to support longer.