What metric are you using for third biggest? The national figures I've seen put the Guardian in a pretty lowly 10th place in terms of circulation. Even if you exclude most of the popular papers (i.e. tabloids) and look only at broadsheets then the Guardian is still only 4th (of 5).

The Guardian certainly punches above it's weight through its internet site the paper itself is nowhere near as big as the other papers.

Source: http://www.theguardian.com/media/table/2013/jul/15/abcs-nati...