What do you do after you beat the 3rd gym in Pokemon LeafGreen?

Answer 1

Go to nugget bridge, battle every trainer there. Then battle the Team Rocket affiliate who will reward you with a Nugget. Move east, or to the right to Bill's House. Free Bill from his freako Pokemon imprisonment and look into his computer. He will give you a ticket to the cruise in Vermilion City. Leave his house and move down and to the left to get out on the opposite side of Nugget Bridge. By that time the officer in the house that was robbed should be out of the way, get in through the door of that house. Battle the Team Rocket affiliate who holds the move DIG. He will give the DIG to you!

When you get to route 9 you wont be able to go any further so go left and down being careful not to jump the ledge which will take you back to Cerulian City. You keep going until you reach route 5. Go all the way down to go to the Underground Path which will take you to Vermilion City. You can battle in the gym now or after you visit the S.S. Anne cruise. If you already competed the gym, the next step is to go to the S.S. Anne, battle Gary and he will tell you the Captain is not feeling well. You should go to the Captain and rub his back. He will reward you with the move CUT.

Go to Digglets Cave and get to the other side. If you keep going to one of the houses in route 2, a Prof. Oak Aide will give you the HM Flash. You can now go back to Digglet's Cave to Vermilion City. Then go back to where you came from (Underground Path) and use CUT on the tree in route 9. Keep going until you reach the Pokemon Center. You will need to teach Flash to one of your Pokemon which you found in route 2.

Once you go through Rock Tunnel you will reach Lavender Town. I completed this game several times but a really nice and thorough walkthrough can be found here:

http://pokedream.com/games/fireleaf/walkthrough/