Congrats on accomplishing so much in your high school experience. Most HS seniors can't post this kind of question because they are still working/grinding on parts of their application that need some attention.
It would be easier if you asked a hypothetical about Brown, Yale, and Princeton because those are more similar being in suburbs of a mid-size city, similar culture, and academics. Cornell, Brown, and Stanford are very different from each other, plus 2 are ED schools and 1 is a SCREA school. Cornell is in the middle of nowhere 4-5 hours from Boston or NY, Brown is less than 1 hour to Boston and 3 1/2 to NYC, and Stanford is 30 min to San Jose and 45 min to SF. While that might not mean a lot for an HS student, when it comes time to for internships, job search, networking, etc to push along your professional career, you don't want to have to relocate each summer or try to do career development from a remote location. The last thing is the culture. Whether you are aware of this or not, Cornell's social life is tied to Greek Life and over 1/3 of students belong to a Frat or Sorority. It's not that dissimilar to Dartmouth. Although there is a good representation of Greek life at Brown and Stanford (11 to 14 houses), it's not the center of social life. Also, 2 are in a snow belt with cold Winters and Stanford is in CA.
Based on the little information you provided which doesn't include your race, gender, location (are you in the US or Int'l), intended major (s), sports (are you a varsity athlete), #of APs or IBs, and their scores, what key "spike"(s) narrative is, academic index (if you know that) and intend career or other preferences, it would very difficult for anyone to embark on this analysis for you.
And if you provided these things, then this post would be a chancing question about which of the 3 schools you have the best probability of getting into based on your chancing profile.
Therefore, I suggest that you fill out a chancing profile on your CV and fill it out as thoroughly as possible and run it against these 3 schools and see what the numbers tell you your % probability of getting into any of them are. I'm pretty confident that Cornell will be the easiest, then Brown and Stanford the hardest. But I don't know what the spread is between the 3 schools. They will clearly not all be a few % percentage points between them, perhaps double digits.
The last thing I would say is that if you get into Cornell or Brown ED, you have to go. This means you'll never know if you would have had a good chance to get into Stanford or not.
Good luck and be reasonable in your expectations of other HS students trying to answer this very personal and complicated question for you. You would be better served to talk to your parents, mentors, and HS college counselor.
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