Don't choose a subject you don't like/aren't good at, i made that mistake with physics (trying to please them) and will be lucky getting a D..
They much prefer you to do well in a subject than choose something you will get a much lower grade at and as you have bio and chemistry it's only cambridge who you won't be able to apply to (although different colleges have different requirements so i may be wrong).
Your current options are very good and show depth of study, don't worry about it, loaads of people only do two sciences and still get offers, it's getting the grades in the first place which counts! don't make my mistake, i spent this year hating physics and college; september can't come soon enough!