From the general impression I am getting, the third interview sounds like the classic good cop/bad cop interview. Some people do those to see how you react to other people in your environment that you may not have an immediate rapport with. I have had this happen to several clients at regional airlines. The people have come off as arrogant, rude, condescending, etc. They are designed to put you off balance to see how you handle yourself in various situations. The key is to see if you can find a common ground and turn it into your favor.
Let me give you an example of how this works in action. When I was an FA, I had a gate agent come down with the prelim paperwork and tell me that there was this really rude woman that was mad because she did not get seat 1D in First Class, but seat 1A instead. My initial reaction was, "oh great!" However, I turned my attitude around and decided that I was going to make her day. (or at least try) When she came down the jetway, I knew immediately who she was. Very gruff, shoving her stuff in the bin, etc. I was very nice to her, got her a drink - still no thawing.
She was dressed in a classic navy suit, with a pin stripe shirt and she had on a tie - except the tie was no ordinary tie. She had on a bow tie with little soldiers and teddy bears on it. (other than this tie, she looked like a woman who thinks she had to dress like a man to be taken seriously)
Just prior to takeoff, I whispered to my 2nd FA, "I bet you $5 that I have her turned around before the end of the flight." The flight was only PHX to BUR, so not a lot of time!
After we got to 10,000 ft., I looked at her and said, "Wow, that is the cutest tie I have ever seen!" Instant thaw! From that point on, we were like best buds and we even exchanged business cards at the end of the flight.
The reason I bring this example up is to illustrate that you can be in control of what happens. You let yourself get off balance and intimidated. When they saw that happening, the interview was over from the start.
I still would have sent a thank you note. You ended up breaking protocol, because you had already sent notes from the first two interviews. Even though you really did not want to work for them, you still need to do that. This business is a very small one, with people moving between airlines all the time. You never know when you may run into the same people again. So, I always tell my clients, in any situation, not to burn bridges, because you never know who will be standing guard at the next bridge you want to cross.
I hope that helps. I would certainly expect that in your interview process, if you have not met with the "head person" yet, be prepared to do so. Companies are trying to make the best decision possible for their business, so the odds of three interviews is very strong. Most companies do them all in one day and rotate the group of individuals between each of the three types of people you stated.
Kathy