How you approach this problem depends completely on what approach was made in the definition of what natural numbers are.

How were they defined in your source work? From the Peano postulates, as a naturally ordered semigroup, or from the Zermelo-Fraenkel axioms?

I expect you already have that is a well-ordered set such that 0 is the "smallest" member, which gives you the answer to 3 straight away.