It varies by school. Most people say 2 science professors and 1 non-science to be safe. Personally, I won't have a non-science professor because I don't really know any, but I will have a supervisor from a non-science/non-medical field.
Also, I'm not sure about this because my school doesn't have a committee letter, but it is my understanding that if you have a committee letter you're good to go, nothing else required.