Continue to play normally with the Pokémon you chose until you get your first 5 Pokéballs. Catch any wild Pokémon in the area around Newbark Town or Cherrygrove City (you need at least one Pokémon in your party at all times). give the starte pokemon a poke-ball it will make your life easier for the next step.
Deposit your starter into the PC at the Pokémon Center, and go to change boxes. While it is saving, turn of your Gameboy and turn it back on again(it helps if you turn it off as soon as it says 'saving'). Be careful when doing this as if you do it improperly, you may mess up your save data.
Load the game as it was before choosing your first starter, but this time choose a different one. Repeat the above steps (catching another wild Pokémon, depositing starter from this game, etc.)
When you load the game a third time, take the starter you didn't pick the other times. Then go to the Pokémon Center and pick up the other two starters from the PC.
Isn't it great when you don't have to choose :)?