This stuff goes back to Biblical times and probably even before that. Certain numbers or combinations of numbers have always been considered "magic" or at least symbolic of various things.
3 is a special number, apparently, getting into many fairy tales, ghost stories, and the Catholic religion.
18 is a special number in the Jewish religion. It represents life, as the first letter of the Hebrew word for life is the 18th letter.
Friday the 13th is, of course, bad luck. Good Friday? 13 at the table for the Last Supper?
7 seems to be special also, though I've never understood why. There's the Biblical explanation of the 7 days in the week, on the 7th day God rested, etc., but I think the magic of the number 7 goes back before the Bible.
Fascinating, and of course I believe that stuff is all a lot of nonsense.