No team in NBA playoff history has ever come all the way back from a 3-0 deficit to win a series, but three teams have come close. The 1951 New York Knicks, the 1994 Denver Nuggets, and the 2003 Portland Trail Blazers are the only teams in league history to force a Game 7 after trailing 3-0 in a series.
---From NBA history.
Now the Celtics have joined them.