How do I address a colleague in an 3-way email with a student? 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?
This answer is a bit meta, I guess. I would probably use a form of address that would seem comfortable to the student, since you are jointly teaching him/her. It would seem to make things like reply-all in an email more comfortable for the student. I'm sure your colleague will understand. If you are being a bit formal with the colleague you can echo that with the student. Ms. James and Prof. Smythe. 

However, some places are first name only and always. Then it would be different. 

One option, of course, is just to ask each person how they would like to be addressed and go with that. You will want to give your own preference as well when you ask. 

Just note that whatever you do, you will be either emphasizing or de-emphasizing the authority structure. Either could be appropriate depending on other factors.