Snopes says that this is unconfirmed, but the rumor persists.

First shalt thou take out the Holy Hammer. Then, shalt thou strike three times, no more, no less.

Three shalt be the number thou shalt strike, and the number of the striking shall be three. Four shalt thou not strike, nor either strike thou two, excepting that thou then proceed to three. Five is right out.

Once the number three, being the third number, be reached, thy pope shall snuff it.