In fact, the underdog streak dates back 42 consecutive games -- to September 30, 2012, when Mike Mularkey was coaching his fourth game with the team.
According to VegasInsider.com, there is no fluctuation based on venue in Vegas. The three points is consistent across the board.
But as @E_Dilla pointed out, Vegas usually gives three points to the home team right off the bat, so it might just be the oddsmakers' way of saying, "Y'all figure this out."
The over/under is set at 41, so if Vegas gets it exactly right the Jaguars will win 22-19 on Thursday. For a team that's scored 22 points or more just three times this season, I'm guessing Vegas is counting on a defensive touchdown.