well, in order to tell what you should do with a 3rd, we really have to decide what the purpose of the norrington's table actually is.
1) A competition between finalist students in the way of sport
2) An actual evaluation of teaching quality, resources and all of the other stuff that goes into enabling a student to do well
If it's 1, make a 3rd 0, a pass -1 and a fail -2 and keep the others the way they are.
If it's 2, take anything at 3rd level or lower out of the equation. The reason is that every college obviously provides the teaching, resources etc. to allow every student to get a 2.2 (probably a 2.1). Those who get 3rds obviously have something wrong with them and don't really reflect any of the qualities that are looked for in reason 2 in each college.