Hey, I'm a Cambridge student. Not a medic but I'm a bio natsci so that's fairly close (we do the same animal physiology practicals). I'd definitely recommend bio, chem and maths. Could you do an AS level too? An essay subject like history might actually be beneficial because medics have to write essays. I got to uni having not written an examined essay since GCSEs because I did all STEM A levels and couldn't write a timed essay to save my life, and I imagine that the medics' essays are similar, so an essay subject could make the first year of uni easier. That being said, it is definitely not necessary. If you only want to do 3 A levels that's completely fine.
The Oxford website says that it requires "Chemistry with either Maths, Further Maths, Biology or Physics." So you could supposedly take chemistry maths and history but I assume that most applicants will have biology too.
This is what the Cambridge website says:
"A Levels in Chemistry and one of Biology, Physics, Mathematics.
Most applicants have at least three science/mathematics A Levels and some Colleges require this and/or particular subjects. See College websites for details.
Please note that in the past three admissions rounds, 95% of applicants for Medicine (A100) offered three or more science/mathematics A Levels and, of these, 23% were successful in obtaining a place. Of the 4% of applicants who offered only two science/mathematics A Levels, just 3% were successful in gaining a place." So Cambridge really wants 3 STEM subjects. And like I said, the physiology module of my natsci course is very similar to some of the content in the medicine course and I don't want to think about how hard physiology would be without biology A level.