second on campus interview for TT faculty position (3rd interview total)
I have been a department chair (math department, large US university) and nonetheless find your situation completely baffling. I have literally never heard of a TT job candidate being invited to a second on-campus interview. This is so unusual that I can’t really come up with a plausible guess for why the department that invited you thinks they need you to visit again; moreover, I think this may potentially bring the competence of the people who came up with this plan into question.

Please don’t rely on idle speculation by strangers online. My advice is to email the people who invited you and ask them to clarify what their plan is for the second visit and why they think it is needed. In my opinion, if you already had one on-campus interview, at least in a normal situation they should have all the information about you that they need in order to make a hiring decision. So I’m guessing their explanation may not be the most logical one. But at least you will have a sense of where they’re coming from.