Can you sit various exams early?

My school offers us to sit the C3 maths exam early, as well as S2 possibly, although there are various places saying some universities dislike you to sit these exams (mainly C3) as it stops you from spreading out your work or something of the sort. I have no problem with sitting it early as we have already learnt the topic and I can get 95% or above in most of the tests, just unsure whether to sit it early if it will impact applying for uni.
-Interested in Natural Sciences at Cambridge for now, if that changes anything.

Yes. I think most universities don't mind but I think Cambridge (and probably other very top unis) tend to want you to be doing at least 3A2s worth of work in your A2 year. So if you're carrying on four into Year 13 then it's no big deal, I'd be hesitant about doing just two full A2s plus C4 (well and S2 of course should you not do it)

What if I were to sit the C3 exam in year 12 and then resit in year 13 regardless of my grade this year, so if something were to go wrong next year then I'd have my grade from this year to 'fall back on', if that's how resits work (just taking the best grade of all attempts at the exam)