How do you deafet the 3rd gym leader in Pokemon platinum?

The third Gym Leader in Pokémon Platinum is Gardenia, the Grass-type Gym Leader of Eterna City. To defeat her, you'll want to use Pokémon that are strong against Grass-type, such as:

* Fire-type: Charmander, Chimchar, Turtwig (after evolving), Ponyta, Magby, and others.

* Flying-type: Starly, Bidoof (after evolving), and others.

* Poison-type: Zubat (after evolving), Croagunk, and others.

* Bug-type: Kricketot, Caterpie (after evolving), and others.

* Psychic-type: Abra (after evolving), and others.

Gardenia's team:

* Cherubi: Level 18, Grass-type

* Turtwig: Level 19, Grass-type

* Grovyle: Level 21, Grass-type

Strategies:

* Prioritize Fire-type: Fire-type moves are super effective against Grass-type, making them your best bet.

* Use Flying-type: Flying-type moves are also super effective against Grass-type, and can be helpful if you don't have a strong Fire-type.

* Watch out for Grovyle: Grovyle can learn the move "Seed Bomb," which is a powerful Grass-type attack. Be careful when using your Fire-type Pokémon against it.

* Use status conditions: Moves like "Thunder Wave" or "Toxic" can weaken Gardenia's Pokémon and give you an edge.

* Level up: Make sure your Pokémon are at a level that is comparable to Gardenia's Pokémon.

Good luck! Let me know if you have any other questions.