I was wondering if anyone had more knowledge on when to submit these 3 types of letters. I know the Thank You letter is optional and doesn't sway much of a decision, but I've seen some friends mix in a letter of interest along with their Thank You letter.

Also, I was planning on submitting an update letter around December time since I applied a bit later in the cycle, but was wondering would I mix in a letter of intent/interest in this same letter too? I just don't want to bombard admissions committees with too many docs, but I will probably have a waiting period in between interviews/decisions and would like to update and reaffirm my interest in certain schools.

TLDR: Don't know the etiquette behind these letters. Should I combine update letters / letters of interest particularly after interviews or do separate letters.