I bet if you post your 3 questions you will get more answers than you will ever need!!
Maybe you could get extra credit for interviewing nurses from 3 different unions- highlighting that all unions are not the same. I think that the question of which union and how that union behaves is more important than whatever specific questions you might ask.
your state and federal guidelines dictate what is an appropriate question and that which is not. keep to the sound interviewing practices you've honed over the years and do not violate the rights of a prospective candidate; simple. or, you get labeled as a union buster. do you have personal indemnity insurance?