Defeat Wallace in sootopolis city, you can only come there by using dive.
You get in right before getting the last badge.
the third badge in sapphire is at mauville city(north of slateport city)
Go to lavaridge town for your next badge.
It is the balance badge where you fight Norman(your father) in Petalburg City .
Petalburg City.
You have to beat flannery
In Sootolis city.
you can get the fith badge by going to petalburg the poke gym there.
In Pokekon Sapphire, you get the 5th badge after defeating your opponent in a Gym Battle at Petalburgh.
You get the third badge in sapphire when you get to Mauville. You have to battle Gym Leader Wattson in the Mauville City Gym.
in lavarage town
You get it in Sootopolis city. If your playing Ruby or Sapphire. You would be fighting Wallace. In Emerald, You fight Juan.