I'm currently doing maths, further maths chemistry and RS for a level (in year 13)!!
For chemistry, if you did triple science, the jump to AS isn't a lot. The jump from AS to A2 is a much bigger, but I wouldn't worry about that less. Some of the material, such as organic, will seem familiar, other areas will be completely new. In terms of workloads, Chemistry definitely has a lesser workload than philosophy. Each board has different content for philosophy/RS but I'd imagine that non matter what board you're doing, it will involve a lot of reading and essay writing. There is certainly a jump between GCSE and level for philosophy. Mainly because what you do at GCSE is very basic and in my opinion quite dry in comparison to a level. If you studied things like the cosmological, design argument at GCSE, you will probably (certainly in the case of RS) revisit those, but study them in more detail. Personally, I love philosophy (going to do it at uni) and so I really think the a level is worth it. However, it is a lot of work so if you think you'll find it interesting, do it!! I also think that with your other subjects, you'll be able to bring some really insightful comments to discussions.
Whilst many people say that you're more likely to get a job with chemistry a level, I think the fact you're doing maths, fm and physics is impressive enough. Also, it can be quite handy to have an essay subject, especially philosophy, because it teaches you how to structure arguments and analyse things in a very particular way.
But ultimately, go for the subject that you'll think you'll have a better time studying - a levels are tough and I think it really helps to love what you're doing
Good luck!!