What are the stages for pursuing medical sciences in America
For a student in the U.S. system, the common academic route to PhD admission in biomedical sciences is to obtain a bachelors degree and then to apply directly for PhD programs. For competitive programs, research experience is very important for applications, and so many applicants do additional research in between their bachelors degree and PhD applications: working in academic labs, participating in post-baccalaureate programs, or sometimes participating in a program that offers a masters degree (particularly if you are taking a more interdisciplinary route), but the masters degree is not usually a strict requirement for a PhD program in biomedical sciences.

Masters degrees are uncommon as a goal in biomedical sciences, and many institutions do not even offer them as a program that accepts applicants. Instead, masters degrees are sometimes awarded to PhD candidates who choose to end their studies early, or they consist of add-on programs (almost like a more developed 'minor') meant to accompany a PhD and are applied for simultaneously. Of course this can vary with the exact institution, and varies in other fields.

For a student outside the U.S. system, things can be more complicated. As discussed in the comments by BrianBorchers, U.S. institutions vary in how they treat bachelors degrees completed in other countries. A 3 year program (an actual 3 year program, not a 4 year program completed early) may not qualify. In that case, your best option may be to get a masters degree in the educational system you got your bachelors degree in, but this is something you should discuss with individual programs you may be interested in applying to in order to find out their specific rules: there is no guideline that will apply to every school or every program. Alternatively, programs may instead have a course of study in mind that you will have to have completed before your application is considered: it will be more important that you can show you have completed those courses in your studies rather than the specific length of your degree.

Most major PhD programs in the U.S. are going to have a web presence with rules and admissions requirements posted, as well as support staff for the program. You should read these as thoroughly as you can, and then ask additional questions to the staff.