It is great that you have an idea in mind for what you want to be when you're older. I studied Sociology A Level AQA and I found it amazing. Don't get me wrong - it wasn't easy, and anyone who says it is has not studied it. There is a lot of information to remember, and you must be able to master essay writing and be able to produce original and interesting arguments and debates.
Sociology would fit in perfectly with your aspirations of going into religion and philosophy, especially if you study the Beliefs in Society unit. I studied Beliefs in Society and it was basically religion in society and the wider picture of it. It is a very rewarding and respected A Level and I definitely would advise you to study it, especially with your other two chosen A Levels, but obviously, the final decision is down to you.