you go the the fourth gym in pastora and battle him
you got to go to veilstone and that's where she is.
You get your 3rd badge in Veilstone city against Maylene.
You can get a badge and TM Shadow Claw after beating the third available gym leader, which is Fantina with the Relic Badge. (She uses Ghost types)
The third badge is in Goldenrod City.
if you are looking for the third gym badge i suggest Veilstone city.
it's the 3rd badge!
pearl
Back in the 3rd town where your dad is hope this helps :)
3rd gym
1.In diamond fantina2.In pearl fantina3.In platinum Wake because fantina is your 3rd Leader
Defeat the 3rd Gym Leader, Wattson, in Mauville City.
The 3rd guy in the elite 4 has the infernape