Much like 5 o'clock shadow, but for those who have just started shaving, and their beard/mustache/sideburns have not become thick enough to develop by 5 o'clock, but 3 days later. It would have the same length after 3 days as a fully developed beard would have by 5 o'clock.
Jim: Hey Bob, you've gotta nice mustache, what'd you grow that today?
Bob: LOL, oh no, its just my 3 day shadow. I've gotta shave it again today.