It seems non intuitive but I think Watson is more likely to win because its competition are so good. If you look at how well it did on Jeopardy here it seems to show Watson is better having humans splitting the points on questions it cannot answer as it is so quick to answer the ones it can.
When the humans buzz in they are right 19/23 about 83% of the time. Suppose Jennings was up against watson and me and I got zero buzz ins. Jennings may have gotten 11 of the buzz ins that Rutter got. Watson seems so fast on the buzzer that it seems to buzz first for nearly every question it could answer 44/50 times it tries to answer it is first.
This means that Jennings could nearly have doubled the number of answers he had but because there are two people who are really good at the sorts of questions only humans could answer these points are divided amongst the humans. Of the 23 human answers only 6 did Watson try to buzz in on.
There is a similar probability puzzle that might help in a three way duel. "You're a cowboy, and get involved in a three way pistol duel with two other cowboys. You are a poor shot, with an accuracy of only 33%. The other two cowboys shoot with accuracies of 50% and 100%, respectively. The rules of the duel are one shot per cowboy per round. The shooting order is from worst shooter to best shooter, so you get to shoot first, the 50% guy goes second, and the 100% guy goes third, then repeat. If a cowboy is shot he's out for good, and his turn is skipped. Where or who should you shoot first?"
Theres another explanation of a three way duel here.
Sometimes having 2 really good competitors increases your chances of winning.