In My notion only "im Urlaub" is commonly in use. The other two variants could possibly be legit. But I do not know anyone expressing "in Urlaub" and I'd say "auf Urlaub" is simply slang. "In Urlaub" as in by now becoming there, appears entirely Improper in Higher-Austrian, but I https://bihlmayermedia-bilder.com/urlaub/wir-sind-im-urlaub/