3rd and 4th Gym leaders.

Third and Fourth gym leaders. The third one has electric type Pokemon. You should use a rock type Pokemon against the third Gym leader. If the third Gym leader has a electric/steel use a fire Pokemon against that Pokemon. Steel is weak against fire type Pokemon. The fourth gym has grass type Pokemon. You should use a fire type Pokemon against the Gym leader. If you used a water type Pokemon you will more likely to loose to the fourth Gym leader. Same has the third Gym leader. After you beat both of the Gym leaders you can go to Fuchsia city to battle the 5th Gym leader. That's all you surpose to know about know the third and fourth Gym leaders.