Instructions
Obtain a PalPad from the basement of a Pokemon Center after you receive your first gym badge.
Access the PalPad from your Key Items inventory section. It will tell you your Friend Code. Write down the code.
Exchange Friend Codes with two other people who play "Pokemon SoulSilver." These people must have the two starter Pokemon you're missing, and they must be willing to trade them to you.
Meet in the basement of any Pokemon Center at a prearranged time, and speak to the woman at the middle desk. You will be taken to a new room, where you will meet up with your friends.
Walk up to your friend and press "A" to trade with him. Trade two of your Pokemon for the two starter Pokemon that you are missing, and you will have all three starters in your possession.