On the one hand, if you are a sophomore you do have more to learn and more time to study to raise your ACT score. On the other hand, general statistics on standardized tests show that the vast majority of students do not raise their score significantly after the second (and certainly not after the third) administration.
Otherwise, I agree that
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Neither of these schools is objectively better than the others - for premed or anything, really - but particularly for pre-med, since medical schools don’t really care where you went to undergrad.
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It’s too early to be making these decisions. Particularly since your current ACT score puts you in the bottom 25% of applicants, and even if you do raise it to a 30, you will be an average applicant in terms of test scores. Miami’s acceptance rate is higher than Emory’s but chances are decent that you won’t really have to make this choice (especially if you are OOS to UNC). There’s also financial aid packages to consider - so basically, you shouldn’t think that hard about this until April of 2017. Just apply to all three.
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Especially if med school is your goal, you do want to go somewhere you can get good grades. Now I don’t think that the weaker applicants always do the worst in the top schools - and you might not be an all-around weak applicant, since we don’t know your grades and such. But I do think that you increase your chances of doing well if you go to a university/college at which you are at least in the middle of the pack.