To do this.. you must start a new game. SAVE at the point where you get the chance to start with your first Pokemon.
choose your first pokemon and read the NOTE (Down from here) then use the 5 pokeballs to catch a Pokemon where you don't care about example: Pidgey,Sentret,HootHoot. put your starter on Pc and change box: SAVING... DON'T TURN
OFF THE POWER. turn it off quickly.. but not too quickly!! if you did it succesfull, you start at the point where you choose a starter, but this time you choose a different one. go to the pokemon center and check the box. if he/she is in it. don't withdraw it! after you get the 5 Pokeballs. use it to catch a "Don't care Pokemon" again. and this time put your 2nd Starter in your pc and change the box again.the last one is very simple. just choose the last starter. and go to the Pokemon Center.. and withdraw the 2 other Starters.now you have Chikorita Cyndaquil an Totodile.

NOTE: After choose a Starter.go to Cherrygrove City on the west side of town. just cross Route 29 and reach the town
from Cherrygrove. you go north to Route 30 and get the Pokedex from Oak. located in a house on Route 30. after that.. go back to Route 29 cause your Rival will run into you and start a fight. after that cross Route 29 to reach New Bark Town cause Prof. Elm wants to speak to you. it's about your Rival. you must give him a name. after that. you get the 5 Pokeballs if you leave the building.