You can't have three 16-0 teams in one conference, the math just doesn't work.
Each team plays 6 division games, 4 games against another division in their conference, 2 games against the other teams that came in the same place as them in the conference, and 4 games against a division from the other conference.
So lets take the recent three AFC undefeated teams from a few weeks back, the Patriots, the Broncos, and the Bengals.
Bengals are in the AFC North, they play the AFC West.
Bronocos are in the AFC West, they play the AFC West
Patriots are in the AFC East, they play the AFC South
So even if it were say the Colts trying to be undefeated and all three teams played their two other games from different positions two of those teams would have to meet each year.