Right. Thank you for the correction regarding Queens'.
However, whether you enter ballot individually or as a group is not to do with whether you can live in the college for three years or not.
All colleges guarantee accommodation at least for three years, and many of them for the duration of the course.
And at many colleges, you usually enter a ballot for a part of accommodation,,whether it's a corridor (or a part of) or a house, so,that you can occupy it with your friends you want to live together.
In some colleges, one person enter the ballot to represent the group (handy if he/she got a first as they have a priority vote) or as a group.
It is possible to enter a ballot on your own to get one available room, but not sure many people actually do it......