Have there been any nba team to win down 3-0?

In the NBA playoffs, no. It has happened once in the MLB (2004 ALCS when Boston defeated the Yankees after being down 3-0) and twice in the NHL (1975 when the Islanders defeated the Penguins in the second round after being down 3-0 and in 1942 when the Maple Leafs defeated the Red Wings in the finals after being down 3-0).