Pokémon Crystal Cheats

Get the 3 starters
You have to use the pokemon duplication cheat with your starter as soon as possible, the only thing is that you need to save before you pick your first pokemon, when you do it pick another stater and go to the PC and the old one will be there, now repeat the same method with the two pokemon then pick the last one as a starter. Now you should have the 3 starters.

Moderator Note: Yes, you can get all three pokemon, but only one of them will register in the Pokedex, so it doesn't count as them ever being caught, which kind of defeats the purpose.