Not all medicine courses require any specific grades in GCSEs and some merely require a token pass at C or above (and Cambridge is in the process of phasing out these GCSE requirements anyway). Additionally A-level Maths is not required by any course, and while useful if it's a problem for you specifically you would be better to take a non-STEM subject as your third option, which is welcomed by some (UCL notably has previously indicated a contrasting subject is considered valuable by their admissions team).
However in almost any circumstance taking A-levels over three years is likely to disadvantage you. While the subject change may make it more acceptable for some, it may be more prudent to take other subjects, excel in those and apply to 6 year courses for "subject changers" or an Access to Medicine course or similar. You may want to contact some of the universities you're considering applying to directly and asking them if they consider students who began non-STEM A-levels but changed to a full set of STEM subjects after a year and then completed those subjects (which were all different from the original ones) in a two year period.