Actually, I disagree completely. First years are load, new and want to get involved in things. Second years are load, don't have exams, and have the time and inclination to run things. Third years are finalists, and as such work hard and want to do well, usually not taking as much time to do extra curricular things.
The first two seem to naturally go together. Put first years and finalists in the same place and the first years will be loud and annoy the 2nd years.
However, if like Univ, you put first and second years in college, and give the third years finalists accommodation where they can study without being annoyed by the younger and less exam-minded, you don't get that problem. Univ is run by second years - the JCR, all college societies, etc. Third years are generally too busy working to run for things. Indeed, I know Trinity does the same, where first and second years live in college, and third years live in other college accommodation.
Moreover, on the finance note, with Univ prices, private accomedation is usually cheaper. I pay £400 a month for my room, which is a little more than most private accommodation.
The finalists could most benefit from peace and quiet and their own space, rather than sharing it with first and/or second years. We don't have the space to have a first year quad, second year quad and third year quad, like Christ Church or Magdalen might, which means either have the third year rooms in and around first and second year ones, or have them living seperately and off site.