Honestly, I would actually advise against taking the 3 sciences and maths. Like others have said before, although it technically doesn't limit you, having a non-science A-level can really show that your well rounded and balanced as an individual, as well as allowing you to see if medicine and sciences are really for you.
When I was in A-levels I did maths, chemistry, biology and sport studies and I am so glad I did. The sports studies was not only a relief from the 'heavier' subjects but also stood me apart from other candidates, something you definitely want when medical schools receive 100s of applications a year.
It also gives you an easy question to prepare for in medical interviews, I got asked about my choice of sports studies at all of my interviews and again, it allowed me to express my passions outside of the classroom. Remember, you'll be going into a competitive enviroment where everyone is exceptional academically, so what you do outside the classroom really matters.
Although, as AtomicMan said, the sciences may help you during your BMAT, that exam is for a minority of medical schools and it wont help you for your UKCAT.
All in all, don't worry too much about your choices, as long as you do biology and chemistry, you will be able to apply. Probably take maths as well (its relatively easy and very useful), and for the last one, do something you enjoy. Your only going to et the opportunity to choose your A-levels once so you may as well include a fun one like music, a language or anything that interests you!
Over the Easter break