It is in Rusturbo City and the leader is Roxanne.
She uses rock type Pokemon like geodude and nsepass such be prepared with a water/grass etc.
behind the first gym
it is a dragon type Pokemon gym
The third gym leader in Pokemon Sapphire is Wattson and he is located in Mauville City.
If you mean Pokemon Sapphire, then it is in the house on the west part of the city with the fifth gym. You have to have defeated the gym leader first.
fire gym in lavaridge
the seventh gym in pokemon sapphire is in mossdeep city.
In sapphire, the 5th gym leader is your dad, in that city after Oakdale.
it is in petalburg near the Pokemon gym
petalburg
In the gym.
You can with AR.
In the lavaridge town gym.