Three deep? You mean three percent of the population.
No,that's a slang expression used 'round my way meaning "far too many".I've also heard the expression used at least once in a Python skit so perhaps it's used 'round your way as well...if only by the Oxbridge crowd.In fact I've heard a lot of words/phrases used in British shows (comedy,drama,etc) that are used by my Irish family.Maybe my ancestors brought them from the Old Country.