I do AS biology, chemistry and maths, and they're not too bad, but it depends on you and your strengths.
Personally, biology is easier than the others, but there is a lot more you need to know and be able to remember.
With the other 2, its really important to actually understand the concept rather than memorise what the textbook says.
Basically, they can be hard initially, and if you don't put the effort in, but if you do actually work for it, do the work and go the extra mile, you shouldn't find it too hard
In terms of making, maths is 100% exam, so that's marked externally.
Science exams are marked externally as well, but depending on exam board, coursework can be marked internally then externally moderated, or may just be marked externally.