Most employers do not typically make you an offer on the spot - even if it is a 3rd interview.

It is probably a series of interviews (that's how my long interviews have been). You'll probably meet with 1-3 people first and then another 1-3 people after that or something.

Or it could be testing, as in typing or a writing sample, etc. I've had to do written tests before at an interview - they gave me a scenario and then I had to type up a plan, draft a press release, etc. right there and then on their computer system.