As above, I am also studying these three subjects, now at A2 level, after having received an A in all of them, and a B in my fourth, French. Do not be deterred by what other people say, if you believe yourself to be capable of managing those subjects, then take them. They are hard work, but manageable, without doubt, and if you enjoy the subjects also, that will make things a lot easier. As for levels of stress, I've found that these subjects have caused me no more stress than I found in my GCSE's. The key is to keep on top of work; to meet deadlines; to conduct as much research around your subject as well as within your subject as you can, and you'll be absolutely fine.