FWIW, the company I currently work for always brings candidates in at least 3 times, and has been known to bring people back as many as 4 and 5 times, not including phone interviews (myself included). They will have you talk to different people, or discuss subjects that they were not satisfied with the first time around. By that time, the interviewee’s nerves should lessen, so they watch for changes in behavior to see how you are most of the time. They tend to focus at first on your skills and then subsequent times if you will fit into the culture. Once you work here, it is nearly impossible to be fired and they don’t do layoffs, so they take hiring pretty seriously. You are not guaranteed a job until they offer it to you - we have brought people back 4 times and yet decided not to make an offer. I would say any position that is paid more than $50k goes through this extended process. I was terribly annoyed by it and almost didn’t take the job at the time, but I understand it now that I work here.
Before I worked here, the third interview was usually time to turn the tables - the third interview was your turn to interview them and ask all the questions about things you care about.