Betting lines are set by bookmakers to try and get the same number of bettors wagering on both teams. This way, they make the most money and their risk is negligible . That's why lines move. More bettors are wagering on one team than the other, so the line is moved to correct for that - drawing bettors to the other side until they have around the same number of bettors on both sides again.
Bookmakers are not trying to predict the outcome of the game. they are trying to make money.
Whether or not you want to use the game as a "predictor" of the outcome, that's up to you.
Just know, that the Bengals being 3 point favorites over the Bills is not a prediction. It's just that the sportsbooks realize they will get the most action on this line - on both sides.
But hey, keep using the word "prediction" if you want, no skin off my nose. But just realize that the Bengals being 3 point favorites does not necessarily mean the books think the Bengals are the better team.