You make one application indicating your three preferred residential colleges.
Your application is initially forwarded to your first preference. If they are unable to make you an offer it will be forwarded to your second preference if they have vacancies. If not, your application will be forwarded to your third preference, as long as they still have vacancies.
If your application does not receive an offer from your three preferred colleges, other colleges with vacancies still available will be invited to consider your application provided that you have checked the relevant box in your application.