I was told by my college personal tutor that it doesn't matter if you take 3, all universities (and med schools) will accept you for 3 now. Having said that, I haven't seen rock solid evidence of this on any website. I'd personally like to see some evidence of this as well before choosing, but I don't know whether I will. I'm doing Chemistry, Biology and Law for my A levels, which I think would still give me a chance in medicine if I choose it in the future (I hope so anyway).
Gosh these linear A levels are a pain!