In my department, professors go by their first names with students and each other. My colleagues in some other departments go by their title and last name. To convey their preference, when referring to such a colleague in conversation with a student, I will use the professor's name and title (e.g., "You should ask Prof. Walker.").
If I am on a 3-way email involving such a professor and a student we are jointly advising, do I address the professor by their first name, title, or not at all?