The third gym is located in Vermilion City it is placed below the Pokemon Center and there is a tree you can Cut nearby it which is the only way to enter it at this time.
The third gym is LT.Surge he has a voltorb pickachu and a raichu I forget what level they are but they are in between 26 to 20
The key to the gym
the 5th gym is saffron gym with sabrina!
The fifth Gym Leader in Pokemon LeafGreen is Koga, the leader of Fuschia City's Gym. He specializes in Poison-type Pokemon and hands out the Soul Badge.
You have to win all of the other gyms then go right into it and the doors are open
Yes, if you get the Gym Badges.
Sabrina's gym in saffron city
there is a key in the Pokemon mansion... you need it to get into the gym... search the Pokemon mansion thoroughly
I think the gym in fuschia city
viridian city
there isn't one
Vermilion city.