After you defeat Gym Leader Brock and get the Boulder Badge, you can go east to Route 3.

Explore Route 3

East of the entrance to Route 3, you can battle Lass Janice. She has a level 6 Oddish. You get 120 Pokédollars and 3 Poké Balls for winning.

Go south from Janice, then east, to find 3 Poké Balls.

Go north from Janice and you will meet a Coach Trainer. You can talk to him to battle Coach Trainer Kareem. He has a level 11 Bulbasaur. You get 1100 Pokédollars for winning, and after the battle, you get a Revive.

Go east from the Coach Trainer and you can battle Youngster Calvin. He has a level 6 Spearow. You get 120 Pokédollars and 3 Poké Balls for winning.

If you go south from Calvin and jump down the ledge, you will find a Potion. From there, you have to go south and west to return to where you were.

Go east from Calvin and you can battle Camper Victor. He has a level 7 Nidoran♂. You get 140 Pokédollars and 2 Great Balls for winning.

South of Camper Victor, you will find some grass. Some of the pokémon you can catch here are Rattata, Spearow, and Ekans.

In this area, you can battle Picnicker Robin. She has a level 7 Nidoran♀. You get 140 Pokédollars and 2 Great Balls for winning.

Go east from Robin and check behind the bush to get a Lure.

Keep going east and you can battle Youngster Ben. He has a level 6 Rattata. You get 120 Pokédollars and 3 Poké Balls for winning.

South of Ben, you can battle Bug Catcher Greg. He has a level 5 Kakuna. You get 60 Pokédollars and 3 Poké Balls for winning.

There is some grass to the east of Greg. You can find a Revive in the grass.

Go north from here to reach Route 4.