found this on
https://sports.stackexchange.com/questi ... e-playoffs
It says it was pretty common when there were only 3 divisions in each conference- pre-78. Since then 7 times, so not as uncommon as I'd have thought.
2006-07 NFC East: Philadelphia Eagles, Dallas Cowboys, New York Giants
2007-08 AFC South: Indianapolis Colts, Jacksonville Jaguars, Tennessee Titans
2007-08 NFC East: Dallas Cowboys, New York Giants, Washington Redskins
2011-12 AFC North: Baltimore Ravens, Pittsburgh Steelers, Cincinnati Bengals
2013-14 AFC West: Denver Broncos, Kansas City Chiefs, San Diego Chargers
2014-15 AFC North: Pittsburgh Steelers, Cincinnati Bengals, Baltimore Ravens
2017-18 NFC South: New Orleans Saints, Carolina Panthers, Atlanta Falcons