In which book does Noam Chomsky refer to a three year old child as
knowing implicitly the formula of his native language? Chomsky, by
using the child example is illustrating his premise that the formula
of language has been encoded in the brain by the time the child is 3
years old.

Hello cocoa and thank you for your question.

The book you are looking for is:
"Powers and Prospects: Reflections on Human Nature and the Socail
Order"

Here is how I found it:

"THE "LANGUAGE FACULTY"

As mentioned before, linguist Noam Chomsky suggests that humans are
born with an innate knowledge of language, and he calls this knowledge
the "language faculty." He invisions this "language faculty" as a
biologically autonomous system in the brain that "has an initial state
which is genetically determined, like. . . the kidney, the circulatory
system, and so on" (13)."
http://www.duke.edu/~pk10/language/psych.htm

The number 13 (above) refers to the page on which the text appears. 
if we look at the bottom of the website we see the book he is
referring to is :

"Chomsky, Noam. Powers and Prospects: Reflections on Human Nature and
the Socail Order. Boston: South End Press, 1996"
http://www.duke.edu/~pk10/language/psych.htm

You can purchase the book from the publishers for $16.00 in paper back
or in Cloth for $40.00
http://www.southendpress.org/order/individual.shtml

and read a little about the book here:
http://www.southendpress.org/books/powers.shtml

OR, you can buy it on amazon.com for $11.20  
http://www.amazon.com/exec/obidos/search-handle-url/index%3Dbooks%26field-keywords%3Dchomsky%20Powers%20and%20Prospects%26url%3Dindex%3Dbooks%26store-name%3Dbooks/104-2351744-5534361

Thank you for your question, and if you need any clarification of my
answer do not hesitate to ask.

Very best regards

THX1138

Search strategy included:
"Noam Chomsky " "year old child"
://www.google.com/search?hl=pt&ie=ISO-8859-1&as_qdr=all&q=+%22Noam+Chomsky+%22+%22year+old+child%22&lr=