You definitely shouldn't be concerned about studying all three sciences with no other variety - plenty of people do it, and it will mean you have a wide choice for subjects at uni anyway - I'd research which universities and courses you might be interested in, but:
Some universities offer Natural sciences (most notably cambridge, durham and Nottingham), where you start off with studying 2 or 3 in the first year, and then specialise properly in the 3rd (at cambridge this is the only route to study sciences other than medicine) and usually you're allowed to take biological options without having done biology at A-level as it's one of the subjects that does change quite a lot at university (there's a much bigger split like organic vs physical chemistry), so you wouldn't be at much disadvantage if you got interested in biology and hadn't chosen it at AS. I don't study it myself, mainly as what I've observed from my friends, biology a level is really content heavy and takes a lot of time and effort to do well in (unlike maths or physics), and unless you really enjoy all aspects (cell biology, zoology, environmental biology - you'll probably have to do the dreaded fieldwork in some dreary place), at some point you'll get fed up.
I'd advise to take the top three subjects that you need to do to study your main interest(s), which is probably maths, physics and chemistry (does your school offer further maths? as that's probably more useful for physics than chemistry), and then choose the subject you most enjoy for your fourth option - a great thing about a level is you get to study what you really like and enjoy, so it would be a shame if you chose biology to make your options slightly wider if you didn't like it as much as something else; remember though that if you do all sciences, your last opportunity to formally study an art (i.e. english) will have ended in year 11 when you're 16 - you'll never have the opportunity to improve (among other things) you're writing skills and techniques.
I'm studying physics, chemistry, english lit. and latin (+ further maths off-timetable) - with the intention of applying to read physics next year. I really enjoyed english this year, it was a nice contrast to the sciences and the texts studied at AS are so much better than GCSE (usually aimed to cover people across the spectrum). Also, my classes actually had different people in them... you'll probably get bored seeing the same faces if you study all three sciences as well.