you need to go to go inside the lighthouse and then you need to get to the fifth floor then you fall down a door and land on the exterior(get a rare candy on the exterior).Then get back inside the lighthouse and go up the ladder.get up to the rest of the ladders and find Jasmine taking care of a sick Pokemon. Talk to Jasmine and she'll ask you to go get some medicine from cianwoods pharmacy.Be careful about the 10 trainers along the way to Jasmine.When you defeat Chuck and get the secret potion, go back to the lighthouse(now you can take the elevator).Give the secret potion to jasmine and she'll give it to ampharos.When the Pokemon is okay, Jasmine will go back to the gym and you can challenge her.
Veilstone city,the gym leader is a fighting type
In Ruby, Sapphire and Emerald, the 3rd Gym Leader is in Mauville City and in Diamond, Pearl and Platinum, the 3rd Gym Leader is in Veilstone City.
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.
in mauville city
At Mavile city
The third gym leader is in Goldenrod City in the radiotower. after you get the radio card she goes to her gym to the east of the radiotower.
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.
Ur 3rd city iz Hearthome city but da gym leader iz actuly not da 3rd gym leader she iz da 5th gym leader.Ur 3rd gym leader iz Maylene in Veilstone city.
She is in Veilstone City.
In Pokémon HeartGold, you can find the 3rd Gym Leader in Goldenrod City.
The third gym leader in Pokemon Sapphire is Wattson and he is located in Mauville City.
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.