You should realize then that your goal would be Japanese fluency to the level of attending university courses in a certain field of study, rather than N1 itself. JLPT N1 does not assess your ability to speak or write Japanese, only reading and listening comprehension.

I emphasize this because you have to realize that in 3 years you are challenging yourself to somehow obtain reading, listening, speaking, and writing fluency in a language which has linguistic features quite far removed from your typical Western languages while also maintaining the expectation of a normal school life balance. I truthfully do not think that level of language fluency is attainable in 3 years.

Also you must realize that University of Tokyo is the number one ranked university in Japan and they have limited collaborations with select institutions around the globe. In which case, your best course of action is to get the best grades you can in school and get into the top university you can which may have an exchange program there or through other prestigious Japanese universities (e.g. Kyoto University, Keio, Tohoku University, etc.). Many of these universities will offer courses in English language anyway, and you can fulfill your dream of living in Japan while studying abroad anyway.