Maths - practice LOTS. Lots of stuff you can use in 'physics and maths tutor'.
Biology - spend time memorising the content using your exam textbook, khan academy (and other resources). Also, use the specification to make sure you have done everything. Also, make sure you look at all the required practical stuff that will come up in the exam.
Comp science - sry I don't do that subject so idk.
It's definitely possible as you still have time to improve. Work hard now and you will get the grades that you want.