Boston has been in that situation before in past NBA Finals, where they were up 2-0 but then lost to the Warriors. Dallas couldn't make their usual shots, and that's the problem. The points coming from Kyrie and Luka weren't easy, unlike the Celtics, where everyone can score. They are good at passing the ball and rotating to defend Dallas's shooters.
Dallas just doesn't have the kind of defense that the Celtics have, and they are heavily reliant on Luka and Irving. Though both played well, it still resulted in the same outcome. They need more than that to win in their last chance in Game 4, but I doubt bettors would still trust them.
It's a big risk for bettors though it might be depending to how bookies will play with the odds, gamblers always look for that and even how big the risk it is as long as the odds attracts them then surely there are still gamblers that will take it, moving back to how the game being played, I agree with you as mentioned Kyrie and Luka provided decent productions but ain't enough to carry and win the game, with the way Boston read Mavs system they continue to have that good rotations to cover any possible open spot for the Mavs, which really help them winning the game, good defense and very balance offense brings them this huge edge, one more win and the series will be over.