How do you get to the third gym leader in pokemon firered?

Answer 1

You must have gained HM01 - Cut in the S.S. Anne and taught it to your pokemon. (After fighting Gary on the second floor, go up to the captain's cabin. He's seasick and puking. Rub his back and he'll give you Cut.)

Cut the tree blocking the path to the gym and go inside, where you'll face a few trainers who use Electric Pokemon. The gym leader has put electric locks on the door that leads to him, so you'll have to solve a puzzle. There are number of trash cans. Press "A" on every one of them until it says that you've found and flipped a switch. You need to flip two switches for the door to open, so you'll need to find the second switch. The second switch is always either above, beside, or below the first one. If you get it wrong, the switch will be reset, and you'll have to do everything over again.

When you do open the doors, you can get to the gym leader. Good luck!