Most unis (except Cambridge and I believe Scottish ones prefer a third science subject but it is not a must) will not at all judge what you take for your third A Level, so pick one you enjoy and can do best in. EPQs are useful to have something to talk about in interviews etc and so are encouraged, but just as a viewpoint of someone who didn’t do one, I didn’t as except Surrey (accepting a C in the third subject, which was my easy A* anyway) no vet school would lower my grade offer from it and I was struggling with Chemistry so preferred to put my energy into that than an EPQ.