I know it hasn't been done very often, but I wasn't sure exactly how many times an NHL team has come back from a 3-0 deficit. The answer? Twice. And it's been a while since it happened:
The 1942 Toronto Maple Leafs did it in the Cup finals, beating the Detroit Red Wings 4-3 to win, at the time, their fourth Stanley Cup.
The 1975 New York Islanders did it in the second round against the Pittsburgh Penguins. The Isles got down 3-0 in the ensuing round against the Philadelphia Flyers, too, and once again forced a Game 7. They lost that one, however.