In what game?
In the goldenrod gym.
In Pokémon HeartGold, you can find the 3rd Gym Leader in Goldenrod City.
The third gym leader is Whitney.
find a way to veilstone to beat maylene
Fantasia is the third gym leader in case you didn't already know that
Sure you can. In ruby, the third gym leader leaves his gym.
It is an HM you get from the third gym leader.
After defeating the third Gym leader
goldenrod radio tower
The third Gym Leader is Lt. Surge and you can find him in Vermilion 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.
The third gym leader in poekmon pearl is maylene, a fighting type gym leader.