As far as I'm aware, the majority of students who get into Cambridge take Maths on top of biochem, so if you're looking to apply there in particular then I'd go for that. But other than Cambridge, the vast majority of universities don't particularly care what your third option is so long as you get a good grade and you're doing bio and chem- including Oxford. It may be worth looking at the websites of the universities you're interested in applying to just to make sure, but I'd just reccommend going for a subject you'll enjoy and you think you'll be able to do well in if you're unbothered about Cambridge.
In terms of EPQ, it also differs from uni to uni. I don't think Oxbridge takes it into account because not every school offers it, so it would be an unfair advantage to some applicants. But some definitely do, for example I have a friend who has got a lowered offer from Exeter if they get an A in their EPQ. It's also a good thing to bring up in interview to demonstrate your interest in the subject if you do a medically-focussed project. It is quite a big time commitment though so you may be better off just focussing on getting the grades. Again, it's worth just taking the time to look at individual university websites or even emailing the admissions departments to see if they take it into account.