umm...you beat the 7th gym
It's behind a house on the way to Snowpoint City. You can't use it though until you get the Icicle badge.
You go to Sunyshore and challenge their gym.
Hearthome's badge
in the 1st gym badge
you get the coal badge after defeating Roark in eterna city
if you are looking for the third gym badge i suggest Veilstone city.
Get the first gym badge
The fifth gym leader in Pokemon platinum is crasher wake but you get the fourth badge.
You can use the GTS in Pokemon Platinum after you get the first gym badge from the Oreburgh City gym.
you must beat the eighth gym leader and he will give you the badge.
Once you obtain a badge it's permanently stored in the badge case.
it's the 3rd badge!