In NFL spread betting, a -3 spread means that a team (the favorite) must win by at least four points to cover the spread.
The underdog can cover the spread either losing by fewer than three points or simply winning the game.
If the margin of victory is three points, it’s a push.
For example: If Patrick Mahomes and the Kansas City Chiefs are -3 against Lamar Jackson and the Baltimore Ravens, the Chiefs must win the game by at least four points.'