From the 2007 playoffs, in an article leading up to a Cowboys-Giants postseason game:
Since the NFL-AFL merger in 1970, teams have played three games in the same season 53 times, but only 17 of those included teams that won both regular-season matchups, according to STATS Inc. In 11 of those 17 games, the team that won the first two games also won the third, the last time by St. Louis over Seattle in 2004.
The Cowboys lost to the Giants in the divisional round, so no sweep there. We can then follow-up by looking at the playoff records on pro-football-reference.com. There have been two opportunities for a 3-game sweeps since. Pittsburgh did it to Baltimore in the 2008 playoffs, and the Cowboys did it to the Eagles in the 2009 playoffs.
To add it all up, 3-game sweeps happened in 13 out of 20 opportunities. That's 65% of the time. So if the Bungs can somehow end up playing Baltimore again in the playoffs, their odds are not great, but not terrible either.
That's just going by history, of course. The true odds depend on many more factors.