Hi and thanks for sharing your profile.
Overall you have a strong profile for the 3 main programs you listed being interested in, and a chance at Stanford and MIT, with Stanford being the most difficult because not only is it one of the most popular programs to apply to with the top candidates applying, it also has a smaller than average class size compared with many of the top programs.
Your GMAT is excellent, your grades strong, and actually having the scientific background helps you stand out as the pool of applicants with experiences like yours is smaller than many. Your experiences combined with the strong scores/stats, including your volunteering, will lend well for both stories and anecdotes to include in your application, and to bring to the classes at business school. 3 years of work experience should be sufficient, and by the time you begin the program you'd have 4. It sounds like you have enough breadth of experiences to draw from in terms of contributing to the class.
At this point I believe that your chances will be strong and the decision will be based on your overall application-- which is primarily your essays at this point since the other areas are covered. So it's important for you to spend a lot of time and effort on those essays in the coming months. You will want to confidently demonstrate what you can bring to the program and what the program will bring to you and to your future career goals. You need to be sure your story makes sense, that your past leads to the MBA which leads to your future goals, and that you also explain why each particular program is right for you, as that can make the difference in some cases.
Feel free to check in with any other questions, and wishing you the best on this process.
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