u need fighting types or ice fang
defeat Whitney the third gym leader
Go defeat the 4th? :)
Defeat the Cianwood city gym leader, then talk to a lady outside the gym and talk to her. She'll give you HM02 (Fly) and if you try asking BEFORE you beat the gym leader she'll not give it to you then, but once you defeat the gym leader.
Defeat the gym leader in cianwood then speak to the girl outside just below the gym.
the third gym is in goldenrod city right after ilex forest
Defeat the 3rd Gym Leader, Wattson, in Mauville City.
After you defeat the gym leader in Goldenrod City, go next door, to the flower shop, and pick up the watering can. After that, head north toward the National Park.
goldenrod radio tower
The gym leaders in Kanto don't have any specific order so there is no "third" gym. It will make things alot easier for people to answer this kind of question if you specify which town the gym is in and not the number as there are alot of possibilities when you just say third. For example, you get to viridian defeat that gym and go to saffron (defeat that gym), then celadon, celadon would then be the third gym. You can also do these in a different order. Then there is also the possibility that you were counting them from the red, blue, yellow perspective and meant the gym in either viridian or cerulean. Please make a new question specifying the town this gym is in and I am sure you will get the answer you are looking for
you have to defeat the gym leader in the 5th gym or something ...
The third gym leader is Whitney.
I get the strongest Pokemon I can get without them disobeying me then I battle the gym leader. I usually focus one or two Pokemon that could defeat the gym leader alone and I train them until I think that they are good enough to defeat the gym leader.