I am definitely happy that I done triple. I know that if I hadn't, this year my physics grade would have brought me down (I haven't got my results yet).
Saying that, there are 7 topics within each of mine, and so that was 21 topcs to learn, as well as the 3 extra exams.
I didn't find it too stressful, only physics. It depends on not just your memory, but how well you're able to apply basic concepts!!
I'm doing Chemistry and biology at 6th form, so thats why I found doing them separately was a lot better, as I feel more confident in my results!!