[SOLVED] number of onto functions

How many onto functions are there from a set with n elements to a set with three elements?

Could somebody explain how to do this, perhaps substituting 5 in for n?

You can solve it by recusive definition: suppose
Let be the number of onto function from a set with n elements to a set with three elements, be the number of onto function from a set with n elements to a set with two elements.
then what is the recusive relation between , and ?

while it is easy to see that .