You can't catch them in the wild but you can use the GTS (Global Trade Station) and trade for one using Wi-Fi.
Turtwig (grass) Chimchar (fire) Piplup (water)
Piplup is stronger, cuter and more loyal than turtwig and chimchar! For example, Piplup, the penguin Pokemon can use water type attacks on chimchar, and ice type moves on turtwig!
in order from best starter to worse Piplup Turtwig Chimchar
if you choose turtwig as your starter than your opponent will have chimchar if you chose piplup then your opponent will have turtwig and if you chose chimchar than your opponent will have piplup.
Piplup, Turtwig, and chimchar
Piplup,Chimchar,Turtwig
Turtwig chimchar piplup
turtwig chimchar piplup
Turtwig Chimchar Piplup
Turtwig, Piplup, Chimchar
You can only get one of these starters in Pokemon platinum: piplup, chimchar, or turtwig. In my opinion it is best to get piplup for type advantage. Example: if you get chimchar your friend (on the game) will get piplup, and chimchar is fire so piplup will win because it is water. Same thing with turtwig, he will get chimchar and grass is weak against fire. But with piplup, piplups only weakness is electricity and niether chimchar, or turtwig is electricity so piplup is your best bet.There is a cheat to get all three of them but I don't know it so the only way you can get piplup if you already have chimchar is to trade with someone who has piplup.
in the begining of the game the profeser will give you three Pokemon and you have to pick one the three Pokemon are chimchar,piplup and turtwig.