The Solution:
The key is to use the "Flash" HM (Hidden Machine). This is a bit tricky because you don't get Flash until much later in the game.
1. Get the "Flash" HM: You can obtain Flash from the "Professor Oak's Lab" in Pallet Town. You'll need to have beaten the Elite Four and become the Champion to get it.
2. Return to 3 Island: Once you have Flash, travel back to the island where the bikers are blocking your path.
3. Use "Flash": Approach the bikers, and use Flash. This will temporarily brighten the area, making it easier to see. While the bikers are distracted, you can quickly slip past them.
4. Save the Girl: Continue forward, and you'll find the girl who was being harassed. You can now rescue her.
Important Notes:
* You might be able to "Flash" past the bikers even without having beaten the Elite Four, but it's not guaranteed. They may be able to move and get back in your way before you can reach the girl.
* The game makes it seem like there's no way to progress, but the "Flash" HM is the intended solution.
Let me know if you have any other questions or need more help with your FireRed journey!