Usually if they ask for AAA it means they want at least an A grade in each subject you take. Whilst in terms of UCAS points A*AB would equate to AAB as would A*A*C, the fact would remain that in one subject, you did not reach the requested standard and strictly speaking the medical school would be under no obligation to accept you. It would then have to boil down to as to whether or not the medical school has places left or if you were very strong at interview and/or if there were factors that affected your performance.
Otherwise given the pressure on places I would say if you don't meet the terms of your offer you won't be attending that medical school.
I would just say work hard and make sure you reach the required standard in each of your subjects.