It would be a deep hole to climb out of. It would be extremely disheartening to be significantly behind after three games. No one in the organization wants to head into the bye week with that thought in mind. The NFL season is too short, too fragile to be desperate after three weeks. The NFL is unlike any other professional sport.
Urgency lives every week.
With every loss, the gravity intensifies the next week. Because of the physical and mental toll, a 16-game slate is all that is afforded coaches and players. Mistakes must be held to a minimum, if the playoffs are to be a serious possibility. In baseball, hockey and basketball, early slumps aren't season-damaging. In football, they're deadly.