Malene has 3 Pokemon (meditite, machoke, lucario) that are all close to lv 24, 25, 26
Veilstone City.
Cherubi (LV. 19) / Turtwig (LV. 19) / Roserade (LV. 22)
it is in veilstone city. hope this helps
Go to Canalave
After beating the third Gym Leader (Maylene the Veilstone Gym Leader) in Pokémon Diamond, you then head to Pastoria City to battle the Gym Leader there who is named Crasher Wake.
Cobble badge
Well,the 3rd gym leader is maylene. She is also gym leader number 3 in diamond and pearl.So no need to worry.
Its really good! I beat the 2nd leader with level 18 nd level 22 pokemon! oull have no problem in winning! You could win the third gym with that team too!
Viel stone cityMaylene is in her gym, you have to complete the simple puzzle within to gain access to her.
to beat the third gym leader use an ground and flying and water and dark type it worked for me trust it!!!!! I pitty da fool!
The third gym in platinum It is diffrent than diamond but it is in Hearthome city she uses ghost type so don't bring normal type happy battling :)
Beat Maylene then, Wake. (Crasher Wake). Then,go to Celestic town. Then, battle her.