In answer to your question, I think that 3 is a better bet than 2 especially if you intend to carry on working throughout, which is what I did. The reasons for this are that it helps break the course in to "bite size" chunks and given that you only have to attend a week (at the most) long revision week per module then it is easier to fit it into work/leave schedules.
i can't speak for the Bristol Groundschool, but from my experience of London Met, they got me through all my exams first time no problem and were supportive and helpful when required.
It is a long hard slog and you have to prepare yourself mentally completely before you undertake the course. I had a start date and finish date clearly in my mind and stuck to it. This meant in the end averaging about 15 hours a week study outside normal work/family commitments. Difficult put not impossible. And by doing it in the three modules it made it easier in my opinion. Hope this helps...