What is the three month rule? Anywhere the use of MDMA is discussed on the internet

The direct origin of the rule seems to be Ann Shulgin, the wife of Alexander (Sasha) Shulgin who rediscovered and spread word of MDMA to a number of therapists.

The only thing that has happened with everyone I know who has used MDMA a great deal is that if they used it more than 4 times a year, they developed a tolerance, and after a few years, it begins to not work for them. They want to get back the magic of the first experience, and that is a mistake, because - as Sasha likes to say -- "You never step into the same river water twice. " I found out, for myself, that using MDMA more often than four times a year was not wise. I used MDMA as a writing drug for about a year. Once a week, I would do a great deal of writing using MDMA; it was a wonderful experience for me to use it that way, but I discovered after that kind of use that I had to go up in dosage to get an effect, and the effect became less and less the magical insight; it was more and more the stimulant effects, and for many years now, I have been totally unable to use it. Not only because now it is illegal, but because I used it once a week, and that was too often. Nobody knew that, then, but now I would advise anyone who wants to use MDMA not to take it more than 4 times a year if you want to continue to get the best effects from it, otherwise you risk losing its effects entirely and permanently.

-Ann Shulgin