Yes, there is a glitch to let you get all 3. It takes advantage of the same programming flaw as the cloning trick. It goes as follows:
1: Start the new game as normal, and save the game just before you choose your starting pokemon.
2: Now play through the game untill you can catch new pokemon (after Elm's aide gives you 5 pokeballs) and catch a new pokemon (any will do).
3: Go to Cherrygrove city pokemon center and desposit your starter (which you can now do as you've caught a new pokemon) in box one. Perform the cloning trick (if you don't know how to do this already, look for it on http://www.gamewinners.com/game_boy_color/PokemonGoldSilver.htm).
4:When restarting the game, you will be back in the Elm lab, and have the previous starter in your PC in box one. You can repeat this trick as many times as desired to get all of the starter pokemon.
WARNING: The cloning trick has been known to cause unforseen glitches in the save file, and in my case once started deleting existing pokemon in the PC box, and I've heard of cases where the whole save file has been corrupted and the save-game lost. So don't overdo it and you'll be fine!
Hope I helped, Jamie.
