Speaking as a college professor who teaches accounting and had taught job-hunting, let me say this.

First of all, let me congratulate you on making it this far!   If you don't get it, then you should still feel good because it's not that you weren't capable, but rather there's somebody with a little more experience.  But either way, making it to the third round is a good sign.

And you are correct in saying that the third interview is to decide if you'll fit with the others.  As far as the questions go, they're most likely going to focus on situations that you may encounter.  And how you might handle different things, or work with certain people.

They've already determined that you can do the job, so your knowledge isn't an issue anymore.  Now it's a question of whether you are the right one for them.  So it comes down more to your personality.  Will those you'll be supervising get along with you?  And will you relate well to the rest of the staff and superiors?  These are now the critical issues that they'll be focusing on.  So I would definitely be prepared to discuss your managment style in more detail.  And how you'd handle your first day on the job.

Finally, you'll be learning more details about what you'll actually be doing, and the environment in which you'll be working.  But their biggest area of focus is how you will interact with others.