Generally speaking, the home team receives a three-point advantage by default when considering the odds and spread. It's like the required height for people to ride a roller-coaster. It'll be there no matter what. After that, the spread is formulated.
So it makes sense that the Bengals are opening the weekend as three-point underdogs, especially if the odds-makers are seeing little that differentiates the Bears and Bengals. And that might be a little more true than not with some picking the Bears (we'll point that out later this afternoon). The over/under is 110.
Regardless, the Bengals might be underdogs by Vegas, but their No. 1 in your head and that's all that matters.