You need to use mr briney's ship to reach slateport city then go north to reach mauville city.
To the leaftnof Mauville
In Mauville City
In mauville city
it is on mauville city
IN MAUVILLE CITY
Go north of Mauville.
You get the third badge in sapphire when you get to Mauville. You have to battle Gym Leader Wattson in the Mauville City Gym.
North of Mauville City
Yes, at Mauville City.
The third gym leader in Pokemon Sapphire is Wattson and he is located in Mauville City.
Go to slate port and then go north
the third badge in sapphire is at mauville city(north of slateport city)