Hi Lauren,
All the best for your HSC. Here is a rundown on the procedure for applying to college at UQ.
You apply to one college using a form (e.g. for Cromwell you can find the form here
Cromwell - Application). Although you apply to only one college you may include two extra preferences on your form.
So if for example you apply with preferences:
1. Cromwell 2. Johns 3. Womens
You will initially be considered by Cromwell and after you have supplied them with your application you will be invited to an interview with the Dean or Head of Cromwell. If then you are unsuccessful with Cromwell they will supply your details to Johns and Womens so that you may be considered by them.
I hope it goes well for you, I would definitely choose Cromwell out of those three (I may be a little biased but I still think they have the best community and I would guarantee you would enjoy your time there). That said Cromwell is super competitive to get into (Johns and Womens are probably similar but I don't know)
Feel free to ask if you had any more questions
Cheers