I have spent considerable time operating all three vehicles you are looking into. Without getting into price and add-ons this is what I feel separates each.
The Kubota has been derived from a tractor and it targeted for heavier work. Its not very refined and the hydraulic drive/steer is a plus when hauling heavy loads and a minus when trying to drive.
The Bobcat is a well balanced vehicle. It has a better drivetrain than the others allowing it to be more capable off-road, able to carry/tow heavy loads that the Kubota can, and not have to worry about high/low or 2wd/4wd gears. Its easier to drive and rides better than the others.
The HPX is much better than previous Gators, and they started this utility vehicle craze. The car looks good and does a lot well, but is outclassed by the others. It doesn't have the power of the Kubota or the features of the Bobcat.