An interview thank-you letter is sent within 24 hours and does one job beyond politeness: it re-argues your fit using something specific from the conversation. Reference an actual problem the interviewer described and add the thought you did not get to finish — that is what makes the note worth reading rather than filing.
Within 24 hours, while the interviewer still remembers the specifics of your conversation.
Email, almost always. It arrives while the decision is live. A printed letter is a nice supplement for traditional industries, never a replacement.
Send each of them a separate note referencing what they personally asked about. Never send the same text to everyone on a panel.