Save the game, then pick one, and then capture any one Pokemon in the grass. Give your starter a Pokeball as a Held Item. Store your starter, let's call it PK1, in box 1 on PC. You should now only have one useless Pokemon in your team.
This is the magic part here:
You switch to box 2, but after it prompts you to save, you turn off the game/reset at a magic moment. The letter F and P, between the word (turn) ofF* + *The (power). You turn it on and if you timed it well, you should be standing before the three Pokémon again.
Pick the next starter, PK2. Go to PC and pull out PK1 and take the Pokeball off it again. Catch another useless Pokémon. Place PK1 & PK2 in box 1. Again you only have a useless Pokémon in your party.
Switch to box 2 while hitting the magic moment as you did prior, and you will again be in front of the trio. Pick last Pokémon, PK3. Go to PC and pull out PK1 & PK2.
You should now have each starter Pokemon, PK 1, 2 & 3. Save the game normally.