I just got done with my 3rd and final interview today. I have sent thank you emails twice before. Is three times too much?
Multiple thank you mails sent with the exact same content may be annoying to the recipient, who might think that you are sending the thank you mail just because you should, but you do not really mean it.
However, other than the above, I have never heard or seen anyone complain about receiving too many thank you mails, if you put some thought into "personalizing" it. Write one or two lines about something that happened specifically during the third interview, such as some information you learned about the company/team or a unique interview question. Don't write an essay about it though.
Moreover, if you are sure this was the final interview, you could also thank them for the overall interview process.
If the interviewer/examiner has sent you an email about the interview, then Ethically, reply is required. but if he didn't, i don't see any need for any Thank-You-Email already