Pep won the Club World Cup with a 3-7-0
Chelsea won PL with a back 3 as well
Not many NT's have employed back 3 tactics in the past because they require training hours that NT coaches don't have. Also most of the players are used to play with back 4's with their teams.
But recently the Back 3 tactics are growing popular in club football and that's why we see NT's like Belgium or England starting to play like that too.