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!
You can get another starter if you get all the sinnoh pokemon. Proffeser rowan will ofer a johto starter.
Global Trade Station
you cant in platinum but you could trade from Diamond and Pearl or another Platinum.
First is 567pokechhskshks
starter Pokemon
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!
check it up on neoseeker.com and you might find it.
Chimchar, piplup, and turtwig.
trade with friends and/or wifi trading
There is no "worst" starter, it's all about your personal preference. They are all good starters. The most challenging is turtwig.
Pick him as your starter
Piplup