Games : How can I select three cadets for my team?

I am just starting out in Final Fantasy Type0 HD, and I have no idea how I can select three specific team members for my team?

So far I have tried in the 'Assignment' menu, but this just seems to set the main party member. Then in the reserve menu I can re-order my cadets, but in neither menu do I see a way to actually build a three cadet party.

How does the party system in Type0 HD work and how can I specify which cadets appear in my team during a battle or when exploring the world.