i) ... a simple constant function works here, because you can use all values of x in the natural numbers, but you just get one value for f(x) which is in the co-domain.

ii) not sure if this one is correct but what if we used , in this case it is mapping the natural numbers to a co-domain that isn't the natural numbers, which is not onto, or one-to-one...

iii) it is just mapping the natural numbers, to the natural numbers, therefore its onto and one-to-one, but you might want to do a formula proof of each of these using the definitions.