I had an interview today, to whom should I send a thank you note?

Hi Reddit,

I had an interview today and am trying to navigate the thank you notes. There were three people present at the interview, and while I would be working with/for all three of them if hired, I would be directly reporting to one of them. She is also the most senior member of their team. Should I send a thank you note only to her, or the other two as well? Is e-mail ok, or should this be handwritten or typed and mailed?

Also, the interview was arranged by a woman at a staffing firm, so should I send her a note? I am inclined to think I should send her a formal thank you only if I were hired for the position, although I could be totally wrong about that. I did leave her a voicemail after the interview (at her request) and made sure to thank her for setting it up.

Thanks for any advice!