Normally, this is only doable via trading. There is a glitch that can be performed to obtain all three, but it requires good timing. Basically, you save in front of the Pokémon before you choose one. Pick one, and then play the game normally until you obtain Pokéballs. Catch a Pokémon, deposit the starter in the PC, change its box, and then shut off the Game Boy while it is saving. When you power on, you will be back in front of the Pokéballs. Pick the next one, and repeat. The Pokemon should now be in the PC.