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!
type this code 010075DA and now u can get all three starter Pokemon
First is 567pokechhskshks
no you can not
check it up on neoseeker.com and you might find it.
Trade
Trade for them.