[Question] Gym battle on equal terms

Hi, I'm making a pokèmon game and I'd like to make the gym leader battle like the anime, that is before the battle you have to choose a default number of pokèmon to take part in the battle. Ex.: the gym leader have 3 pokèmon and before the battle start you have to choose 3 pokèmon in your team and than you can use only the 3 choosen pokèmon in that battle, but i don't know how to do it, can somebody help me please?
(sorry for my english i'm not very good at it)