You can choose to type a letter in a business format on a letter-sized piece of paper, or you can personally handwrite a note on a plain fold over card.  If you have personalized stationary, this is a good time to use it.
Many employers appreciate the personalization of a handwritten note; however, if your handwriting is less than legible, do not attempt a handwritten note.  Additionally, when choosing to type or handwrite your note, consider your rapport with each of the people with whom you spoke.  You may wish to type a note to an individual with whom your relationship seemed more formal and send a personalized note to an individual with whom you developed a more casual rapport.  Avoid emailing your thank you note if possible.  If you have missed the three-day timeline and have not yet put a note in the mail, you may email a note instead.   Email is always considered to be more casual and because it takes less time to send an email, it may be construed to have less impact.