How do you get turtwig chimchar and piplup on Pokemon platinum?

You can get them. Piplup will be in Lake Verity, Turtwig will be in Lake Valor, and Chimchar will be in Stark Mountain. Or, you can choose them as your first Pokemon. Or, you can trade them with a buddy.

this is actually wrong, those are the legendary lake trio in the lakes, the only way you can get all 3 is A) trade, B) Cheats to get them or C) get cheat to catch other people's Pokemon and find the trainers with them