It's happened only a few times. This was on MSNBC:
"None of the previous 25 major league teams to fall behind 3-0 even forced a series to seven games. The wild-card Red Sox became only the third of 239 teams in the four major North American sports leagues to overcome a 3-0 deficit in a best-of-seven series and win, joining hockey’s 1942 Toronto Maple Leafs and the 1975 New York Islanders."
"None of the previous 25 major league teams to fall behind 3-0 even forced a series to seven games. The wild-card Red Sox became only the third of 239 teams in the four major North American sports leagues to overcome a 3-0 deficit in a best-of-seven series and win, joining hockey’s 1942 Toronto Maple Leafs and the 1975 New York Islanders."