i'm in your conscience
The glitch in Gold and Silver (I am fairly sure it was fixed in Crystal) to get all three starters goes like this (if memory serves):
1. Save in front of Elm before getting your starter.
2. Pick your starter, doesn't matter who.
3. Get to the point to where you have Pokeballs.
4. Go catch a junk Pokemon and then give a Pokeball to your starter and box it, then switch boxes.
5. When it's saving and all the words (Saving, do not turn off the power) appear (ALL of them have to be there), turn off the game.
6. Turn it back on and you pick another starter. Rinse and repeat as many times as desired. It'll be easier if you give your starter that Pokeball the first time around.