you have the grades at GCSE which shows now much effort you seem to be putting in, doing a science or math is one of the hardest graded Alevel subject but as long as you are willing to put the work in and effort even if you think they are too hard when you start them then go for it. Im sure you will do well.
if you think it may be too hard when you start then take one away and only do three, and if you school makes you do four then choose an easier subject to balance out the work (I chose photography as my easier subject)
hope this helps