as a year 12 (going into year 13) who is also an aspiring medic planning to apply to cambridge if i get a decent ucat
please listen to what other people here are saying. i feel like you're focusing too much on the goal.
work hard for a-levels (as i'm sure you will), get at least A*A*A predicted, then work hard for UCAT and do well in that. once you have all your stats, and you're sure you can/want to apply to cambridge, because things do change, then dream big, aim high and go for it.
i feel like at this point in time you're subconsciously too fixed on the goal of cambridge/ucl. it's great to aspire for that and i feel like its motivating you to do well. but don't overfixate on it.
also i agree with the others that there isn't much point in resitting those three gcses for all the reasons mentioned by them, but in the end that's up to you
good luck with year 12 and i hope things go well for you