Get All 3 Starter Pokemon From Prof Elm:

GameShark Code: 0100BAD7 << Zeros not the letter.

*If the code is ON, and you are just starting a game, once you get to the point of choosing the starter, keep picking all of them. You will now possess all three starters. Also, the first Pokemon you choose, your Rival will still get the Pokemon with the type advantage (i.e. Cyndaquil > Totodile) even if you took all of them.