It may be worth considering that, for 2017, 2018 and 2019 entry, 98% of successful applicants took Biology, 97% took Mathematics and 96% took Chemistry. There will also be some people who took Physics as their 3rd Maths / Science subject. Not taking a third, if successful, would put you in a very small minority.
Also, you mention that loads of people in your school do four A-Levels. I don't know if that's because your school is just large (i.e. there are loads of people who do four, but many many more who do fewer) or that you go to a particularly high-achieving school but they will also consider your school type and the average grade gotten by your peers.
Finally, why are you so set on Cambridge? You haven't even started your A-Levels and you're already debating whether or not it's worth adding an extra A-Level you don't really want to do to make your application stand out.