What should I ask deans during my interview?
You want to be friendly, of course, and make a good impression. Depending on the school, these meetings are not usually too grueling; one purpose of the meetings is just to put a face with your name. There is a good chance these meetings will end up being shorter than the scheduled time. 


Be prepared to give a very brief "elevator style" summary of your work. 
The deans may ask you about grant funding. You may have a good sense, depending on your field, how important grants are to you getting the position. (Andy W mentioned this in the comments). 


Some particular questions I would ask include:


I would ask all three: Where do you see the program/college/university going in another 10 years?  This can help you tell whether your vision for a school fits with theirs. 
I would ask the Dean of Science about tenure practices in the college. Do your homework and read the tenure policies before you arrive, of course. As long as you are polite and non-pushy, you can ask for clarifications for anything that isn't clear. You can ask about tenure rates in general, but don't ask anything that would require talking about individual candidates.  If you are in a field where grants are important, you should also be sure you know the weight given to grants when they make tenure decisions. 
If you will need anything unusual (e.g. large start-up equipment costs), you should mention that to a departmental representative first, and they can advise whether it needs to be mentioned to the dean. 
If you are trying to arrange a second hire for a spouse, the dean may be able to discuss that. Again, you can talk with the department first.  CAUTION: This bullet only applies to applicants who are also interested in finding a job for their spouse, and who have decided to bring up their spouse before getting an offer. There isn't room in this answer to get into the debate about whether it is preferable to wait until you have an offer to mention that your spouse is looking for a job, which is another commonly advocated strategy for applicants looking to find a job for their spouse at the same institution.