There are only two way to get all three1.Have a friend trade with you for them2.Use a hack or Action Replay
you cant in platinum but you could trade from Diamond and Pearl or another Platinum.
trade with friends and/or wifi trading
You can get another starter if you get all the sinnoh pokemon. Proffeser rowan will ofer a johto starter.
Global Trade Station
You have to choose one then trade it with a friend, start again get a different starter trade this one to a friend, then start again chossing the only other starter Pokemon get the other two back from your friends and you have all three!
Start with the first starter Pokemon on leafgreen. If you are okay with deleting your data, then this will work. Have the first Pokemon and six others in the PC, and transfer your Pokemon onto your platinum game. Then once that is done, and you are all done catching the transferred Pokemon, then save the game. Eject the platinum game card. then when you are going onto the gameboy game, press "new game". Choose a different starter Pokemon, and do the same thing as the first, but have six Pokemon in bthe PC. When you are don't with that, do the same as before. Eventually you will have all three starters from leafgreen on your platinum!
First is 567pokechhskshks
type this code 010075DA and now u can get all three starter Pokemon
check it up on neoseeker.com and you might find it.
Well this is one way you will need two Nintendo ds's and a platinm and diamond or pearl or platinum, just get a starter Pokemon in diamond in pearl or diamond and trade it into platinum, repeat this until you have all starters! If you have an action replay though, You can go to trainers who have the Pokemon, battle them, then capture their Pokemon!
there is no other way unless you either trade or go to the global trading center in jubilife
no you can not