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 :)
1.In diamond fantina2.In pearl fantina3.In platinum Wake because fantina is your 3rd Leader
3rd gym
Defeat the 3rd Gym Leader, Wattson, in Mauville City.
It depends which ones are you looking for. The 8th Badge is in Sootopolis, The 7th badge is in Mossdeep, The 6th Badge is in Fortree, The 5th badge Is your dad in emerald it is found at Petalburg, the 4th Badge is in Fallabore, The 3rd badge is in Mauville, The 2nd badge is in Dewford, The 1st badge is in Rustboro.