Can I apply to US universities with a 3 year high school transcript?
It is unclear what you mean. If you attend HS in a country where HS is typically three years (10, 11, 12), then you apply with that transcript. That said, you will apply before your year 12 grades are available, so you typically list your classes and later report your grades (at midyear point in Feb and/or at the end in June or July). Some colleges dont ask for 9th grade transcript if your country typically counts 9th grade as part of upper middle school. It varies by university and by country. Ask the university to which you are applying. If your question is can I graduate HS a year early and apply to colleges when I am a junior? - The answer is generally yes, but again, it depends on the HS and the college. You probably need to finish all your HS requirements to get a diploma, and you probably need a diploma to be admitted to college. But in some cases, you can complete all the requirements in three year and graduate early.