At some point, it will happen. An NBA team will lose the first three games of a playoff series and win the next four. Such teams are 0-150, but someone sometime will do it.
On paper -- and paper buys you precisely zero wins -- the Boston Celtics are the best candidate in history to pull off the improbable. Most teams who fall behind 3-0 are huge underdogs in the first place. Only nine prior top-two seeds have ever trailed a series 3-0 to a team seeded No. 3 or worse, according to the gurus at ESPN Stats & Information. The Celtics might be the strongest of those 10 top-two teams to fall so far behind; they are the only one to finish No. 1 or No. 2 in both points scored and allowed per 100 possessions in the regular season.
The Miami Heat have the worst résumé of the lower-seeded teams who seized 3-0 leads over those 10 teams. They are the only one with a negative regular-season point differential. At No. 8, they are the lowest seed to go up 3-0 on a top-2 seed, per ESPN Stats & Information. The previous "worst seed" to snare such a lead: the 1994-95 Houston Rockets, who won the first three games of the NBA Finals over the Orlando Magic -- the East's No. 1 seed -- and completed the sweep. Those Rockets were an atypical No. 6 seed; they were defending champions, and reinvigorated their season with a mega-deal for Clyde Drexler.
Most of the other series in this sample were either clashes of equals (conference semifinals between No. 2 and 3 seeds) or series in which the nominal lower seed was the better team and favorite: the 2001-02 Los Angeles Lakers facing the No. 1-seeded New Jersey Nets in the Finals, and the 2017-18 Cleveland Cavaliers (LeBron James' last Cleveland team) upending the No. 1-seeded Toronto Raptors.
These Heat shoving the Celtics to the brink so fast (and so rudely) marks an anomaly. If you asked a computer to simulate the perfect circumstances for the first 3-0 comeback, it would produce something like this.