The answer to that depends on what you're asking. Students DO apply to schools after completing 3 years, but they are expected to complete their degrees.
Many traditional 4 year med schools have wording that would allow them to accept students who have not obtained a Bachelor's, but in reality, it doesn't happen. In the very instances that it does, the acceptees are non-traditional students with a great deal of unique experience that makes it possible.
There are some schools that take students before they have completed their degrees, and later confer both a bachelor's and doctorate. I am very unfamiliar with these programs and will leave it to others with better understanding to explain those programs.