At my son's school, it was a mixture. They started the 3 science subjects in year 9, so they were definitely 3 year courses, but GCSE options were chosen mid year 9 to start year 10, so basically, all pupils did the first year of a 3 year GCSE course for the sciences, but many would drop a science at the end of year 9 and just continue with 2.
As said above, Maths and English are longer term anyway, so there wasn't really a formal "start" to the GCSE courses.
History was interesting as it was basically a 5 year course starting in year 7. Right from the first week at secondary, the pupils were doing exam style questions, i.e. "to what extent", etc - which was quite a culture shock I seem to remember as the pupils were more used to factual history questions such "what did was the magna carta signed", so quite a challenging first year really. Whether by accident or design, they also did a lot of the subject teaching in years 7-9 which formed the GCSE course, i.e. Richard & John, Crusades, Roses, Richard III, Normans, Henry, Elizabeth & the Charles, etc. With the core knowledge having been taught in years 7-9, they didn't spend much time in years 10 & 11 on the traditional British history topics so spend a lot of time on the other modules, such as Middle East and Weimar, so you could say that was a 5 year course too, especially as they didn't need to go back to basics for the exam question technique either.
Geography was a 2 year course and they didn't actually finish the course and had no time for revision either. That was with a super-organised teacher who barely missed any lessons, so it just highlighted how much content there was, basically too much for just 2 years. Apparently, they now do more of the core topics in years 7-9, so they don't have a 3 year course, but some of the content is covered before the formal teaching of the GCSE starts.