Also in terms of difficulty, if you understood chemistry gcse then you can also come to understand a-level chemistry although it is quite hard to fully understand sometimes. I don't do biology but to my knowledge there is lots and lots of memorization and a huge variety of topics. Chemistry is nicely split into physical, inorganic and organic chemistry which do have links between each other and it feels like one subject that builds on itself gradually. Biology on the other hand has lots of disconnected areas like say habitat and DNA but I'm sure you'll be fine if you do some good revision. I have no knowledge of psychology I'm afraid.