go into the pokémon contest place, in the north of the city, and talk to fatina, the gym leader, you should recognise her by here big purple hair, she should then go into her gym, then it will be open.
You use your strong(est) Pokemon to kill the Gym Leader's Pokemon.
it's the 3rd badge!
The third Gym is the Gym in Veilstone City. You can get there by looking on your map. The leader specializes in Fighting Pokemon.
I have Pokemon platinum the 3rd gym leader always their your probaly just not lucky.
Just the way between Vielstone (3rd Gym) to Pastoria(4th Gym)
With your pokemon. (Flying or Psychic are the best types for Fighting pokemons).
if you are looking for the third gym badge i suggest Veilstone city.
where can i find the second gym in Pokemon platinum
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.
The Gym Leader of the Snowpoint Gym.
The water Pokemon gym leader is in Pastoria City.