I took triple science and loved it. Science and maths is really my forte at school so my GCSE grades are better than they would have been had I taken double science. I am planning to take Chemistry, Biology, Physics and Maths at AS level.
If you are planning to do A levels in a science subject then triple science would probably be better as going into your A levels you will have more knowledge than a person who has only done double. However if you feel that your grades in science would suffer because you were trying to do too much then I suppose it's better to do less but do it well.