without maths its possible you'll be at a disadvantage because chemistry expecially relies on differentials, which is maths you learn in as level maths. however students can do well if they put the extra work in. these courses differ widely from GCSEs as content is more intense. if you're willing to put in the work there's no reason why you shouldn't get a chance to study medicine.
one thing i will say is though, it's easier to drop a subject than pick it up. so you can go for maths and see where it goes; what your personal tutor advises when you're there.
medicine looks at your predicted grades. even if you get A*s at A level, if you got Cs in AS level, for example, you'll most probably not get accepted. just to aware you
as a third subject, maybe go for something that will highlight skills that medicine look for. research, for example, is a big part of any science. History helps improve your research and general communication skills a lot. English is great but perhaps doesn't show your skills as much. i recomend, since the history a level course doesn't differ as much from GCSE, and from your predicted grades, that history would be a good choice. i enjoy history at a level anyway.