Your problem with taking 2-3 years to complete a GCSE, is that the syllabus is likely to change over that time. Your exam and later assessed coursework may cover topics which you haven't been taught. I tried doing an archaeology GCSE by distance learning part-time, and that was the trap I got caught in. The teaching, marking and online elements were great, but the syllabus changed at the end of my first year. To have actually taken the exams in the following year to get the qualification, I would have had to re-start the course.