Prove the Axiom of Pair using: "for any A and B, there is a set C such that A is in C and B is in C."

Prove the Axiom of Union using: "for any S, there exist U such that if X is in A and A is in S, then X is in U."

Prove the Axiom of Power Set using: "for any set S, there exists P such that X is a subset of S implies X is in P."

I would really appreciate any input. Thanks!