For Sapphire, Ruby and Emerald The third gym is an electric type, so use Swampert to fight it. If u did not choose Swampert, at least do not use water type or flying type.
level 30
The third gym leader is in Hearthome City. But i think the gym leader says that you are not ready yet. (don't remember) you have to beat Pastoria and Veilstone's Gym leader first and then you can battle Hearthome's leader.
You have to go to Veilstone City and go to the gym. Then you battle gym trainers and go past them. You will get to Maylene, the third gym leader. You can battle her. Unless you're talking about the Hearthome Gym. Fantina is the leader of that gym. If that's what you meant, you need to fight the Veilstone Gym and the Pastoria Gym first.
After beating the third Gym Leader (Maylene the Veilstone Gym Leader) in Pokémon Diamond, you then head to Pastoria City to battle the Gym Leader there who is named Crasher Wake.
you have to break the code
The third gym leader is Whitney.
The third Gym Leader in Pokémon HeartGold who is Whitney the Goldenrod City Gym Leader, she'll be in the Radio Tower when you arrive in the city. You have to get the Radio Card for the PokéGear before you can battle her.
You get the third badge in sapphire when you get to Mauville. You have to battle Gym Leader Wattson in the Mauville City Gym.
Fantasia is the third gym leader in case you didn't already know that
In order to battle the third Gym Leader Korrina in Pokémon X and Y, you must first battle all the trainers inside the Gym. Then you'll be able to rail over there with your rollerskates.
You have to Dive and then you can battle the Gym Leader.
Beat Maylene then, Wake. (Crasher Wake). Then,go to Celestic town. Then, battle her.
Sure you can. In ruby, the third gym leader leaves his gym.