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.
Veilstone city,the gym leader is a fighting type
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.
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.
in mauville city
At Mavile city
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.
The third gym leader is Whitney.