Route 3 is a grassy path that connects Pewter City in the west with Mt. Moon in the east, although technically part of the area before Mt. Moon is Route 4. The first part of the route is nothing but a series of Pokemon Trainers back to back.
Defeat Calvin to receive P120 and 3 Poke Balls. After the battle, jump the ledge to the south to find a Potion. Jump the next ledge to find 3 Poke Balls at the bottom. Return to the north side of the route after collecting the items. Follow the path east to the next Trainer.
Defeat the Coach Trainer to receive P130 and TM57 Pay Day. Further north is a Pokemon Center. Make sure you pick up the Escape Rope on the left side of the building before entering.
Inside the Pokemon Center is a man who will sell a Magikarp for P500. Normally this would be considered outrageous, but you won’t be able to catch a Magikarp until you reach Fuchsia City, and this particular Magikarp has very high stats naturally. I suggest buying this one, but save before you do so. You can always reset if you want a different nature.
That’s all there is for Routes 3 and 4. Enter Mt. Moon to continue your adventure.