Verbal short hand for WWW, the term was coined from the phrase Vee-Dub, being the popular name for the VolksWagon Bug."Vee-Dub bug"
So when verbalizing (Saying) a web address it is faster and flows better saying "Three-Dub Dot whatever dot com" Vs saying "double-you double-you double-you dot whatever dot com."
(standing in person) "The site address is three-dub dot psyberwolf dot com"
Saying "double-you double-you double-you" is now shortened to saying "Three-Dub"