biology is a hard a-level, yes, but the same can be said for history. if you enjoy it and put the work in you might not find it as difficult. if your natural talent is essay based subjects doing 3 of those may be no problem for you- some people do manage to do it and get good results. even if you're not interested in pursuing any scientific in the future it's nice to have a mix of subjects that require different skills and it'll probably keep some doors open if you do end up changing your mind about whatever you wanna do later.