I'm doing advanced maths, physics and chemistry this year plus higher biology. My advice is to take the three advanced highers, they complement each other and if your teachers have said you are capable you should manage. You will probably need to work harder in sixth year to maintain your grades from fifth year - and be prepared to see your advanced highers through even if you get an unconditional because it won't look good to future employers if you drop out or don't do as well as you could. The projects do require a lot of time and effort, but if you get the experiments done early you should be able to do your write-up justice. Maths does require a lot of practise at home as there is a lot of content to cover so there probably won't be so much time for questions in class time, however Maths and two sciences is less commitment than three sciences (fewer projects). I know I do an extra subject and I also commit a lot of time to running inter house events etc. (I'm a house captain and prefect, but most people in sixth year will probably be involved with something between yearbook committee, running clubs/societies etc.) but the idea that sixth year is 'fun' is a bit misleading - amidst UCAS applications and the stress of projects/dissertations, my year in general is probably under more stress than in fifth year despite a lot of people having unconditionals. i don't know what your school's position is but at our school if you take three advanced highers you only need to do three subjects as opposed to the standard four in sixth year, so you will probably have quite a few free periods to do stuff in - and for most of the year you will be able to use these for leisure so don't worry about missing out on spending time in the common room (if you have one) and the banter which goes aong with it!